What’s the Bottom Line?

What do we know about the effectiveness of complementary approaches for flu and colds?

• No complementary health approach has been shown to be helpful for the flu.
• For colds:
  • Complementary approaches that have shown some promise include oral zinc products, rinsing the nose and sinuses (with a neti pot or other device), honey (as a nighttime cough remedy for children), vitamin C (for people under severe physical stress), probiotics, and meditation.
  • Approaches for which the evidence is conflicting, inadequate, or mostly negative include vitamin C (for most people), echinacea, garlic, and American ginseng.

What do we know about the safety of complementary approaches for colds and flu?

• People can get severe infections if they use neti pots or other nasal rinsing devices improperly. Tap water isn’t safe for use as a nasal rinse unless it has been filtered, treated, or processed in specific ways.
• Zinc products used in the nose (such as nasal gels and swabs) have been linked to a long-lasting or even permanent loss of the sense of smell.
• Using a dietary supplement to prevent colds often involves taking it for long periods of time. However, little is known about the long-term safety of some dietary supplements studied for prevention of colds, such as American ginseng and probiotics.
• Complementary approaches that are safe for some people may not be safe for others. Your age, health, special circumstances (such as pregnancy), and medicines or supplements that you take may affect the safety of complementary approaches.

Some Basics About Flu and Colds

Each year, Americans get more than 1 billion colds, and between 5 and 20 percent of Americans get the flu. The two diseases have some symptoms in common, and both are caused by viruses. However, they are different conditions, and the flu is more severe. Unlike the flu, colds generally don’t cause serious complications, such as pneumonia, or lead to hospitalization.

No vaccine can protect you against the common cold, but vaccines can protect you against the flu. Everyone over the age of 6 months should be vaccinated against the flu each year. Vaccination is the best protection against getting the flu.

Prescription antiviral drugs may be used to treat the flu in people who are very ill or who are at high risk of flu complications. They’re not a substitute for getting vaccinated. Vaccination is the first line of defense against the flu; antivirals are the second. If you think you’ve caught the flu, you may want to check with your health care provider to see whether antiviral medicine is appropriate for you. Call promptly. The drugs work best if they’re used early in the illness.

What the Science Says About Complementary Health Approaches for the Flu

No complementary approach has been shown to prevent the flu or relieve flu symptoms.

Complementary approaches that have been studied for the flu include the following. In all instances, there’s not enough evidence to show whether the approach is helpful.

• American ginseng
• Chinese herbal medicines
• Echinacea
• Elderberry
• Green tea
• Oscillococcinum
• Vitamin C
• Vitamin D

What the Science Says About Complementary Health Approaches for Colds

The following complementary health approaches have been studied for colds:

American Ginseng

• Several studies have evaluated the use of American ginseng (Panax quinquefolius) to prevent colds. A 2011 evaluation of these studies concluded that the herb has not been shown to reduce the number of colds that people catch, although it may shorten the length of colds. The researchers who conducted the evaluation concluded that there was insufficient evidence to support the use of American ginseng for preventing colds.
• Taking American ginseng in an effort to prevent colds means taking it for prolonged periods of time. However, little is known about the herb’s long-term safety. American ginseng may interact with the anticoagulant (blood thinning) drug warfarin.

Echinacea

• At least 24 studies have tested echinacea to see whether it can prevent colds or relieve cold symptoms. A comprehensive 2014 assessment of this research concluded that echinacea hasn’t been convincingly shown to be beneficial. However, at least some echinacea products might have a weak effect.
• One reason why it’s hard to reach definite conclusions about this herb is that echinacea products vary greatly. They may contain different species (types) of the plant and be made from different plant parts (the above-ground parts, the root, or both). They also may be manufactured in different ways, and some products contain other ingredients in addition to echinacea. Research findings on one echinacea product may not apply to other products.
• Few side effects have been reported in studies of echinacea. However, some people are allergic to this herb, and in one study in children, taking echinacea was linked to an increase in rashes.

Garlic

• A 2014 evaluation of the research on garlic concluded that there isn’t enough evidence to show whether this herb can help prevent colds or relieve their symptoms.
• Garlic can cause bad breath, body odor, and other side effects. Because garlic may interact with anticoagulant drugs (blood thinners), people who take these drugs should consult their health care providers before taking garlic.

Honey

• Honey’s traditional reputation as a cough remedy has some science to back it up. A small amount of research suggests that honey may help to decrease nighttime coughing in children.
• Honey should never be given to infants under the age of 1 year because it may contain spores of the bacterium that causes infant botulism. Honey is considered safe for older children.

Meditation

• Reducing stress and improving general health may protect against colds and other respiratory infections. In a 2012 study funded by the National Center for Complementary and Integrative Health (NCCIH), adults aged 50 and older were randomly assigned to training in mindfulness meditation, which can reduce stress; an exercise training program, which may improve physical health; or a control group that didn’t receive any intervention. The study participants kept track of their illnesses during the cold and flu season. People in the meditation group had shorter and less severe acute respiratory infections (most of which were colds) and lost fewer days of work because of these illnesses than those in the control group. Exercise also had some benefit, but not as much as meditation.
• This study is the first to suggest that meditation may reduce the impact of colds. Because it’s the only study of its kind, its results shouldn’t be regarded as conclusive.
• Meditation is generally considered to be safe for healthy people. However, there have been reports that it might worsen symptoms in people with certain chronic physical or mental health problems. If you have an ongoing health issue, talk with your health care provider before starting meditation.

Probiotics

• A 2015 evaluation of 13 studies found some evidence suggesting that probiotics might reduce the number of colds or other upper respiratory tract infections that people catch and the length of the illnesses, but the quality of the evidence was low or very low.
• In people who are generally healthy, probiotics have a good safety record. Side effects, if they occur at all, usually consist only of mild digestive symptoms such as gas. However, information on the long-term safety of probiotics is limited, and safety may differ from one type of probiotic to another. Probiotics have been linked to severe side effects, such as dangerous infections, in people with serious underlying medical problems.

Saline Nasal Irrigation

• Saline nasal irrigation means rinsing your nose and sinuses with salt water. People may do this with a neti pot (a device that comes from the Ayurvedic tradition) or with other devices, such as bottles, sprays, pumps, or nebulizers. Saline nasal irrigation may be used for sinus congestion, allergies, or colds.
• There’s limited evidence that saline nasal irrigation can help relieve cold symptoms. Studies of this technique have been too small to allow researchers to reach definite conclusions.
• Saline nasal irrigation used to be considered safe, with only minor side effects such as nasal discomfort or irritation. However, in 2011, a severe disease caused by an amoeba (a type of microorganism) was linked to nasal irrigation with tap water. The U.S. Food and Drug Administration (FDA) has warned that tap water that is not filtered, treated, or processed in specific ways is not safe for use in nasal rinsing devices and has explained how to use and clean these devices safely.

Vitamin C

• An evaluation of the large amount of research done on vitamin C and colds (29 studies involving more than 11,000 people) concluded that taking vitamin C doesn’t prevent colds in the general population and shortens colds only slightly. Taking vitamin C only after you start to feel cold symptoms doesn’t affect the length or severity of the cold.
• Unlike the situation in the general population, vitamin C does seem to reduce the number of colds in people exposed to short periods of extreme physical stress (such as marathon runners and skiers). In studies of these groups, taking vitamin C cut the number of colds in half.
• Taking too much vitamin C can cause diarrhea, nausea, and stomach cramps. People with the iron storage disease hemochromatosis should avoid high doses of vitamin C. People who are being treated for cancer or taking cholesterol-lowering medications should talk with their health care providers before taking vitamin C supplements.

Zinc

• Zinc has been used for colds in forms that are taken orally (by mouth), such as lozenges, tablets, or syrup, or used intranasally (in the nose), such as swabs or gels.
• Oral Zinc
  • A 2012 evaluation of 17 studies of various types of zinc lozenges, tablets, or syrup found that zinc can reduce the duration of colds in adults. Two evaluations of three studies of high-dose zinc acetate lozenges in adults, conducted in 2015 and 2016, found that they shortened colds.
  • Some participants in studies that tested zinc for colds reported that the zinc caused a bad taste or nausea.
  • Long-term use of high doses of zinc can cause low copper levels, reduced immunity, and low levels of HDL cholesterol (the “good” cholesterol). Zinc may interact with drugs, including antibiotics and penicillamine (a drug used to treat rheumatoid arthritis).
• Intranasal Zinc
  • The use of zinc products inside the nose, such as gels or swabs, may cause loss of the sense of smell, which may be long-lasting or permanent. In 2009, the FDA warned consumers to stop using several intranasal zinc products marketed as cold remedies because of this risk.
  • Prior to the warnings about effects on the sense of smell, a few studies of intranasal zinc had suggested a possible benefit against cold symptoms. However, the risk of a serious and lasting side effect outweighs any possible benefit in the treatment of a minor illness.

Other Complementary Approaches

In addition to the complementary approaches described above, several other approaches have been studied for colds. In all instances, there is insufficient evidence to show whether these approaches help to prevent colds or relieve cold symptoms.

• Andrographis (Andrographis paniculata)
• Chinese herbal medicines
• Green tea
• Guided imagery
• Hydrotherapy
• Vitamin D
• Vitamin E

NCCIH-Funded Research

NCCIH-supported researchers are conducting a variety of studies relevant to the flu or colds.

Topics of recent research include:

• Whether probiotics can enhance older adults’ response to flu vaccine
• Meditation and exercise for preventing acute respiratory infection
• How psychosocial factors, such as the quantity and quality of social relationships, influence the likelihood of catching colds
• The effects of echinacea on the immune system.

More To Consider

• Complementary health approaches should never be used as a substitute for flu vaccination.
• Tell all your health care providers about any complementary health approaches you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care

A healthy lifestyle offers many benefits, including helping to prevent heart disease, type 2 diabetes, obesity, and other chronic diseases. Another important benefit is that healthy routines enhance your immunity.

The immune system is the body’s way of protecting itself from infection and disease; it fights everything from cold and flu viruses to serious conditions such as cancer.

Our immune systems are complex and influenced by many factors. Vaccines build immunity against specific diseases. Some additional ways you can strengthen your immune system are eating well, being physically active, maintaining a healthy weight, getting enough sleep, not smoking, and avoiding excessive alcohol use.

Eat Well

Eating well means emphasizing plenty of fruits and vegetables, lean protein, whole grains, and fat–free or low–fat milk and milk products. Eating well also means limiting saturated fats, cholesterol, salt, and added sugars.

Eating well provides multiple nutrients that support optimal immune function. Be aware, however, that too much of some vitamins and minerals can be harmful. Talk to your health care provider if you think you need nutritional supplements.

Be Physically Active

Regular physical activity helps you feel better, sleep better, and reduce anxiety. Combined with eating well, physical activity can help a person maintain a healthy weight.

Following the physical activity recommendations for your age provides immediate and long–term benefits. Emerging research also suggests that physical activity may potentially benefit immunity.

Maintain a Healthy Weight

Excess weight can affect how your body functions. Obesity, defined as a body mass index (BMI) of 30 or more in adults, is linked to impaired immune functions. Obesity may also lower vaccine effectiveness for numerous diseases, including influenza, hepatitis B, and tetanus.

Safe ways to help maintain a healthy weight include reducing stress, eating healthy foods, getting enough sleep, and engaging in regular physical activity.

Get Enough Sleep

Scientific evidence is building that sleep loss can negatively affect different parts of the immune system. This can lead to the development of a wide variety of disorders.

Quit Smoking

Smoking can make the body less successful at fighting disease. Smoking increases the risk for immune system problems, including rheumatoid arthritis.

Avoid Too Much Alcohol

Over time, excessive alcohol use can weaken the immune system.

In Summary

Immunity is your body’s defense against foreign organisms. Taking care of yourself will help your immune system take care of you.

Oral zinc lozenges may reduce the duration of the common cold when started within 24 hours and taken for a time period of less than 2 weeks. Intranasal zinc has been linked to a severe and permanent loss of smell and should not be used.

What Does the Research Show?

Several randomized controlled trials and systematic reviews and meta-analyses have been conducted to examine the effects of zinc on the common cold.

• A 2015 meta-analysis of three randomized trials on zinc acetate lozenges for the common cold (dosages of 80–92 mg/day) found that zinc acetate lozenges shortened the duration of many common cold symptoms such as nasal discharge, nasal congestion, sneezing, sore throat, cough, and muscle ache. However, no differences in duration of headaches and fever were seen. The authors concluded that zinc acetate lozenges (at doses of about 80 mg/day) may be a useful treatment for the common cold, started within 24 hours of symptom onset, for a time period of less than 2 weeks.
• A 2013 review found that use of oral zinc reduces the duration of colds when taken within 24 hours of symptom onset and that prophylactic use of oral zinc reduces the incidence of colds, school and work absences, and antibiotic prescriptions. A 2012 systematic review and meta-analysis of 17 randomized controlled trials involving a total of 2,121 participants found similar results.
• A 2012 review of the treatment of the common cold in pediatric patients concluded that evidence supports the use of oral zinc as a possible treatment and prevention of the common cold in children, although its use may be limited by the need for frequent administration and potential side effects, including bad taste, throat irritation, nausea, and diarrhea.

Safety

• Oral zinc can cause nausea and other gastrointestinal symptoms.
• Long-term use of zinc, especially in high doses, can cause copper deficiency and may increase the risk of urinary tract problems, reduce immune function, and cause other side effects.
• Zinc may interact with drugs, including antibiotics and penicillamine.
• The intranasal use of zinc can cause anosmia (i.e., loss of the sense of smell), which may be long-lasting or permanent. In 2009, the U.S. Food and Drug Administration warned consumers to stop using several intranasal zinc products marketed as cold remedies because they had been linked to cases of anosmia.

For most people, vitamin C does not prevent colds and only slightly reduces their length and severity. Vitamin C is generally considered safe except when taken in high doses.

What Does the Research Show?

Several randomized controlled trials and a few reviews, including a Cochrane review, have examined the effects of vitamin C for treating or preventing the common cold.

• A 2013 Cochrane review of results from 29 clinical trials involving more than 11,000 people found that taking vitamin C regularly (at least 0.2 grams per day) did not reduce the incidence of colds in the general population but may be useful for people exposed to brief periods of severe physical exercise (e.g., marathon runners, skiers, and soldiers training in subarctic conditions). Some studies have shown that vitamin C is associated with modest reductions in the length and severity of cold symptoms; however, these results were not replicated in the few therapeutic trials that have been conducted.
• A 2012 review of treatment of the common cold in children and adults found that prophylactic vitamin C modestly reduces the duration of cold symptoms, but not the incidence of colds, in both children and adults.

Safety

• Vitamin C is generally considered safe; however, high doses can cause digestive disturbances such as diarrhea, nausea, and abdominal cramps.

Although there is the potential that some preparations of echinacea are more effective than placebo for treating colds, the overall evidence for clinically relevant treatment effects is weak. Results of individual prophylaxis trials consistently show positive (though not significant) trends, although potential effects are of questionable clinical relevance.

Echinacea products vary widely containing different echinacea species, plant parts, and preparations. The many clinical trials of echinacea for colds have also varied widely, in terms of products studied, research methods, and study results.

What Does the Research Show?

Several reviews, including a Cochrane review, of many randomized controlled trials mostly in adults have examined the effects of echinacea for preventing or treating colds.

• A 2016 systematic review and meta-analysis found that echinacea might have a preventative effect on the incidence of upper respiratory tract infections, but it is unclear whether the effect is clinically meaningful. In addition, there was no evidence that echinacea changed the duration of upper respiratory tract infections.
• A 2014 Cochrane review of 24 double-blind randomized controlled trials involving 4,631 participants concluded that echinacea products have not been shown to provide benefits for treating colds. Although there is the potential that some preparations are more effective than placebo for treating colds, the overall evidence for clinically relevant treatment effects is weak. This same review also concluded that the results of individual prophylaxis trials consistently show positive (though not significant) trends, although potential effects are of questionable clinical relevance.
• A 2013 review concluded that therapeutic use of Echinacea purpurea may improve cold symptoms in adults, but the evidence is inconsistent. The review also concluded that prophylactic use of echinacea preparations is ineffective for preventing the common cold. The effects of echinacea in children are uncertain; only a small amount of research has been done in children, and the results of that research are inconsistent.

Safety

• Few side effects have been reported in clinical trials of echinacea; however, some people may have allergic reactions.
• In a 2003 randomized controlled trial conducted in children, use of Echinacea purpurea was associated with an increased risk of rash.

Currently, not enough research has been conducted to determine whether probiotics may prevent colds, and little is known about their long-term safety.

What Does the Research Show?

Several randomized controlled trials on a variety of strains and doses, as well as reviews and meta-analyses, have been conducted to examine the effects of probiotics on the common cold.

• A 2015 Cochrane review of 13 randomized controlled trials involving 3,720 participants including children, adults, and older adults found that probiotics (any specified strain or dose) were better than placebo in reducing the number of participants experiencing episodes of acute upper respiratory tract infections (URTI), the mean duration of an episode of acute URTI, antibiotic use, and cold-related school absences. These findings suggest that probiotics may be more beneficial than placebo for preventing URTIs; however, the quality of the evidence was low or very low.
• A 2014 systematic review and meta-analysis examined the effectiveness of probiotics on the duration of illness in healthy children and adults who develop common acute respiratory infections and found evidence from “good-quality” randomized controlled trials that probiotics reduce the duration of illness in otherwise healthy children and adults.

Safety

• As pointed out in a 2011 Agency for Healthcare Research and Quality assessment of the safety of probiotics, little is known about the long-term effects of probiotics.
• It appears that most people can use probiotics without experiencing any side effects—or with only mild gastrointestinal side effects such as gas—but there have been some case reports of serious adverse effects, and the risk of serious side effects may be greater in people with underlying health conditions. Probiotics should not be used by people with serious underlying health problems except with close monitoring by a health care provider.

Nasal saline irrigation (e.g., using a neti pot) may have benefits for relieving symptoms of the common cold in children and adults, and may have potential benefits for relieving some symptoms of acute upper respiratory infection.

What Does the Research Show?

Several randomized controlled trials in both children and adults, as well as a recent Cochrane review, have been conducted to examine the effects of saline nasal irrigation on the common cold.

• A 2015 Cochrane review of five randomized controlled trials involving 544 children and 205 adults found that saline nasal irrigation possibly has benefits for relieving the symptoms of acute upper respiratory tract infections; however, the trials included in the review were generally too small and had a high risk of bias.
• A 2012 review in children concluded that during acute illness, saline nasal irrigation can help alleviate sore throat, thin nasal secretions, and improve nasal breathing, and can reduce the need for nasal decongestants and mucolytics.

Safety

• Nasal irrigation is generally safe; however, neti pots and other rinsing devices must be used and cleaned properly.
• Most important is the source of water that is used with nasal rinsing devices. According to the U.S. Food and Drug Administration, tap water that is not filtered, treated, or processed in specific ways is not safe for use as a nasal rinse. Some tap water contains low levels of bacteria and protozoa, including amoebas, which can stay alive in nasal passages and cause potentially serious infections.
• Improper use of neti pots may have caused two deaths in 2011 in Louisiana from a rare brain infection that the state health department linked to tap water contaminated with an amoeba called Naegleria fowleri.

Research suggests that buckwheat honey is superior to placebo for reducing frequency of cough, reducing cough, and improving quality of sleep for children with the common cold. Honey should not be used in children younger than 1 year of age because of the risk of botulism.

What Does the Research Show?

There have only been a couple of small studies in children that have examined the effects of honey for symptoms of the common cold.

• A 2010 study, involving 139 children aged 2 to 5 years, compared the effect of honey, dextromethorphan, and diphenhydramine on nighttime cough and sleep quality and found that receiving a 2.5-mL dose of honey before sleep had a more alleviating effect on upper respiratory infection-induced cough compared with dextromethorphan and diphenhydramine doses.
• A 2007 study of 105 children aged 2 to 18 years with upper respiratory infections compared honey to similar pharmacologic agents and found that parents rated honey most favorably for symptomatic relief of their child’s nighttime cough and sleep difficulty due to upper respiratory tract infection.

Safety

• Honey should not be used in children younger than 1 year of age because of the risk of botulism.

Geranium extract (Pelargonium sidoides) may be helpful in relieving symptoms of acute bronchitis, acute sinusitis, and the common cold in children and adults, but the quality of evidence is low.

What Does the Research Show?

Several randomized controlled trials in adults and in children, and a recent Cochrane review, have been conducted to examine the effects of geranium extract on the common cold.

• A 2013 Cochrane review of eight randomized controlled trials involving adults and children concluded that Pelargonium sidoides may be effective in relieving symptoms in acute bronchitis in adults and children, and sinusitis in adults. Also, Pelargonium sidoides may be effective in alleviating symptoms of acute rhinosinusitis and the common cold in adults, but there is some doubt. The overall quality of the evidence was considered low for main outcomes in acute bronchitis in children and adults, and very low for acute sinusitis and the common cold.
• A 2007 double-blind, randomized controlled trial in 103 adults with common colds found that Pelargonium sidoides (administered in liquid drops 3 times a day) significantly reduced the severity of symptoms and shortened the duration of the cold compared with placebo.

Safety

• Research suggests that Pelargonium sidoides is generally well tolerated in most people.

A recent Cochrane review concluded that there is insufficient clinical trial evidence regarding the effects of garlic in preventing or treating the common cold.

What Does the Research Show?

Only a few clinical trials have been conducted to examine the effects of garlic supplements on the common cold.

• A 2014 Cochrane review of a single randomized controlled trial, involving 146 participants who received either a garlic supplement or a placebo for 12 weeks, concluded that there is insufficient clinical trial evidence regarding the effects of garlic in preventing or treating the common cold. The single trial suggested that garlic may prevent occurrences of the common cold but more studies are needed to validate this finding.

Safety

• Garlic is probably safe for most people in the amounts usually eaten in foods.
• Taking garlic supplements may increase the risk of bleeding. Patients on anticoagulants should use caution when considering garlic supplements.
• Garlic has been found to interfere with the effectiveness of some drugs, including saquinavir, a drug used to treat HIV infection.

Some preliminary research suggests that elderberry may relieve symptoms of flu or other upper respiratory infections.

What Does the Research Show?

A small number of studies in people have evaluated elderberry for flu and other upper respiratory infections; however, conclusive evidence from high-quality clinical trials is lacking.

• A 2020 review of studies concluded that although there is a hypothetical benefit of elderberry for the treatment of viral infections, there isn’t enough conclusive evidence from high-quality clinical trials.
• A 2019 meta-analysis of four studies found that supplementation with black elderberry (Sambucus nigra) was significantly effective at reducing the total duration and severity of upper respiratory symptoms, compared with placebo.
• A 2016 double-blind, randomized controlled trial evaluating elderberry in air travelers suggested that elderberry may reduce cold symptom duration and severity.

Safety

• Raw unripe elderberries and other parts of the elder tree, such as the leaves and stem, contain toxic substances (e.g., sambunigrin) that can cause nausea, vomiting, and diarrhea; cooking eliminates this toxin. Large quantities of the toxin may cause serious illness.
• Little is known about whether it’s safe to use elderberry during pregnancy or while breastfeeding.

It’s that time of year again—cold and flu season.

Each year, approximately 5 to 20 percent of Americans come down with the flu. Although most recover without incident, flu-related complications typically lead to at least 200,000 hospitalizations and between 12,000 and 60,000 deaths each year. Colds generally do not cause serious complications, but they are among the leading reasons for visiting a doctor and for missing school or work.

Some people try natural products such as herbs or vitamins and minerals to prevent or treat these illnesses. But do they really work? What does the science say?

1. Vaccination is the best protection against getting the flu. Starting in 2010, the Federal Government’s Centers for Disease Control and Prevention has recommended annual flu vaccination for all people aged 6 months and older.
   
   There is currently no strong scientific evidence that any natural product is useful against the flu.

2. Zinc taken orally (by mouth) may help to treat colds, but it can cause side effects and interact with medicines. Zinc is available in two forms—oral zinc (e.g., lozenges, tablets, syrup) and intranasal zinc (e.g., swabs and gels). A 2015 analysis of clinical trials found that oral zinc helps to reduce the length of colds when taken within 24 hours after symptoms start. Intranasal zinc has been linked to a severe side effect (irreversible loss of the sense of smell) and should not be used.
   
   A note about safety:Oral zinc can cause nausea and other gastrointestinal symptoms. Long-term use of zinc, especially in high doses, can cause problems such as copper deficiency. Zinc may interact with drugs, including antibiotics and penicillamine (a drug used to treat rheumatoid arthritis).

3. Vitamin C does not prevent colds and only slightly reduces their length and severity. A 2013 review of scientific literature found that taking vitamin C regularly did not reduce the likelihood of getting a cold but was linked to small improvements in cold symptoms. In studies in which people took vitamin C only after they got a cold, vitamin C did not improve their symptoms.
   
   A note about safety:Vitamin C is generally considered safe; however, high doses can cause digestive disturbances such as diarrhea and nausea.

4. Echinacea has not been proven to help prevent or treat colds. Echinacea is an herbal supplement that some people use to treat or prevent colds. Echinacea products vary widely, containing different species, parts, and preparations of the echinacea plant. Reviews of research have found limited evidence that some echinacea preparations may be useful for treating colds in adults, while other preparations did not seem to be helpful. In addition, echinacea has not been shown to reduce the number of colds that adults catch. Only a small amount of research on echinacea has been done in children, and the results of that research are inconsistent.
   
   A note about safety: Few side effects have been reported in clinical trials of echinacea; however, some people may have allergic reactions. In one large clinical trial in children, those who took echinacea had an increased risk of developing rashes.

5. The evidence that probiotic supplements may help to prevent colds is weak, and little is known about their long-term safety. Probiotics are a type of “good bacteria,” similar to the microorganisms found in the body, and may be beneficial to health. Probiotics are available as dietary supplements and yogurts, as well as other products such as suppositories and creams. Although a 2015 analysis of research indicated that probiotics might help to prevent upper respiratory tract infections, such as the common cold, the evidence is weak and the results have limitations.
   
   A note about safety:Little is known about the effects of taking probiotics for long periods of time. Most people may be able to use probiotics without experiencing any side effects—or with only mild gastrointestinal side effects such as gas —but there have been some case reports of serious side effects. Probiotics should not be used by people with serious underlying health problems except with close monitoring by a health care provider.