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NCCIH Clinical Digest

for health professionals

The Flu, the Common Cold, and Complementary Health Practices:
What the Science Says

February 2012


Woman sneezing

© BananaStock

Complementary Health Practices for the Flu

Scientific Evidence

  • Annual vaccination is the best protection against contracting 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.
  • Although a few studies suggest a potential role for some complementary therapies, there is no strong scientific evidence that any complementary health practice is useful against the flu.
  • Examples of complementary health practices that have been studied for the flu include:
    • Chinese herbal medicines
    • Dehydroepiandrosterone (DHEA)
    • Echinacea
    • Elderberry
    • Green tea
    • N-acetylcysteine (NAC)
    • North American ginseng
    • Oscillococcinum
    • Pomegranate extract
    • Vitamin C
    • Vitamin D.

The following resources provide more information for health care providers on the flu:

Complementary Health Practices for Colds


Zinc has been used for colds in oral forms (such as lozenges, tablets, or syrup) and intranasal forms (such as swabs or gels). Oral zinc may help to treat colds, but it can cause side effects and interact with medicines. Intranasal zinc has been linked to a severe side effect and should not be used.

Scientific Evidence

  • A 2011 systematic review of 15 clinical trials of oral zinc, involving more than 1,300 people, concluded that zinc helps to reduce the length and severity of the common cold in healthy people when taken within 24 hours after symptoms start. The review also concluded that zinc, taken at low doses for at least 5 months, reduced the number of colds in children.
  • The dose, formulation, and length of use that would be most helpful for colds while producing the fewest side effects have not yet been established.


  • 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 (a drug used to treat rheumatoid arthritis).
  • The intranasal use of zinc can cause anosmia (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.

Vitamin C

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.

Scientific Evidence

  • A 2010 systematic 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 likelihood of getting a cold but was associated with modest reductions in the length and severity of cold symptoms.
  • However, in five trials in people who were exposed to extreme physical stress (marathon runners, skiers, and soldiers training in subarctic conditions), taking vitamin C cut the number of colds in half. In studies in which people took vitamin C only when they had a cold, the vitamin did not consistently decrease the length or severity of their colds.


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


Echinacea has not been proven to help prevent or treat colds. It usually does not cause side effects, but allergic reactions and rashes have been reported.

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.

Scientific Evidence

  • Overall, the scientific evidence on echinacea for colds is inconclusive. There is limited evidence that some echinacea preparations might reduce the length or severity of colds in adults, but other preparations did not seem to be helpful.
  • Four NCCAM-funded clinical trials of echinacea for colds all indicated that echinacea did not reduce the length or severity of cold symptoms.
  • Echinacea has not been shown to reduce the number of colds that adults catch.
  • 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.


  • 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.


The evidence that probiotics may help to prevent colds is weak, and little is known about their long-term safety.

Scientific Evidence

  • Although a 2011 systematic review of 10 clinical trials involving more than 3,000 people 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.


  • As pointed out in a 2011 Agency for Healthcare Research and Quality assessment of the safety of probiotics, little is known about the effects of taking probiotics for long periods of time.
  • 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.
  • Research on the safety of probiotics is ongoing.

Other Complementary Health Practices

Current evidence does not support recommending certain complementary health practices that have been investigated for colds, such as:

  • Andrographis (Andrographis paniculata)
  • Astragalus (Astragalus membranaceus)
  • Chinese herbal medicines
  • Elderberry (Sambucus nigra)
  • Garlic (Allium sativum)
  • Green tea
  • Guided imagery
  • Honey
  • Hydrotherapy
  • Nasal irrigation
  • North American ginseng (Panax quinquefolius)
  • Stress management
  • Vitamin E.

NCCIH Clinical Digest is a service of the National Center for Complementary and Integrative Health, NIH, DHHS. NCCIH Clinical Digest, a monthly e-newsletter, offers evidence-based information on complementary health approaches, including scientific literature searches, summaries of NCCIH-funded research, fact sheets for patients, and more.

The National Center for Complementary and Integrative Health is dedicated to exploring complementary health products and practices in the context of rigorous science, training complementary health researchers, and disseminating authoritative information to the public and professionals. For additional information, call NCCIH's Clearinghouse toll-free at 1-888-644-6226, or visit the NCCIH Web site at NCCIH is 1 of 27 institutes and centers at the National Institutes of Health, the Federal focal point for medical research in the United States.


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This page last modified April 21, 2016