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NCCIH Clinical Digest

for health professionals

Myth-Busting Popular Natural Products Marketed for Disease Prevention and Wellness: What the Science Says

January 2017

Clinical Guidelines, Scientific Literature, Info for Patients: 
Myth-Busting Popular Natural Products Marketed for Disease Prevention and Wellness

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Improving Sleep Problems

Myth: Herbs such as valerian, chamomile, and kava are effective for insomnia.

Fact: Various herbs such as valerian, chamomile, and kava, and homeopathic medicines sometimes used as sleep aids have not been shown to be effective for insomnia, and important safety concerns have been raised about a few. For example, kava supplements have been linked to a risk of severe liver disease.

However, there is evidence to suggest that using relaxation techniques, such as progressive relaxation, guided imagery, biofeedback, or deep breathing exercises, before bedtime can be helpful components of a successful strategy to improve sleep habits. Current evidence also suggests that melatonin may be useful in treating several sleep disorders, including jet lag, delayed sleep phase disorder, and sleep problems related to shift work.

What Does the Research Show?

The available evidence on efficacy of relaxation techniques for sleep disorders consists of several small randomized controlled trials, systematic reviews, meta-analyses, and specific recommendations in independent clinical practice guidelines.

  • Relaxation techniques. A 2006 systematic review of 37 small preliminary studies suggested that behavioral therapies, such as cognitive behavioral therapy, including relaxation techniques, were associated with changes in several sleep parameters of participants with primary insomnia or insomnia associated with medical and psychiatric conditions. Other psychological therapies were also assessed.
    • Relaxation was one of five treatments that met criteria for empirically-supported psychological treatments for insomnia. This review was conducted by a task force commissioned by the American Academy of Sleep Medicine (AASM) in order to update its practice parameters on psychological and behavioral therapies for insomnia.
    • The accompanying 2006 practice parameter report published by the AASM suggested that cognitive behavioral therapy appears to be effective (although studies incorporated in the review were small and not designed to assess efficacy). Other psychological and behavioral interventions, including relaxation training and stimulus control therapy may be similarly effective. The Report recommended inclusion of at least one of these interventions in initial treatment strategies for chronic primary and comorbid (secondary) insomnia.
  • Melatonin. A 2007 practice parameter report issued by the AASM includes the recommendation of melatonin supplements to promote daytime sleep among night shift workers. Studies that support this recommendation found that melatonin administration prior to daytime sleep after night work shifts improved daytime sleep quality and duration, and caused a shift in circadian phase in some but not all participants. It did not enhance alertness at night. AASM 2015 clinical practice guidelines(link is external) for the treatment of intrinsic circadian rhythm sleep-wake disorders recommend strategically timed melatonin (for the treatment of delayed sleep-wake phase disorder, blind adults with non-24-hour sleep-wake phase disorder, and children/adolescents with irregular sleep-wake rhythm disorder and comorbid neurological disorders), and light therapy with or without accompanying behavioral interventions (adults with advanced sleep-wake disorder, children/adolescents with of delayed sleep-wake phase disorder, and elderly with dementia and irregular sleep-wake rhythm disorder).
    • The AASM guidelines also include the recommendation of melatonin supplements to reduce symptoms of jet lag and improve sleep following travel across multiple time zones. A 2002 Cochrane review (link is external) supporting this recommendation assessed the effectiveness of oral melatonin for alleviating jet lag after air travel across several time zones, and concluded that melatonin is remarkably effective in preventing or reducing jet lag, and that occasional short-term use appears to be safe.
    • 2013 meta-analysis of 19 randomized placebo-controlled trials involving 1,863 participants with primary sleep disorders found evidence of small but statistically significant improvements in sleep onset latency (reduction in sleep latency of 7.06 minutes), total sleep time (increased total sleep time by 8.25 minutes), and overall sleep quality with melatonin use. These effects do not appear to dissipate with continued melatonin use. The investigators concluded that although the absolute benefit of melatonin compared to placebo is smaller than other pharmacologic treatments for insomnia, melatonin may have a role in the treatment of insomnia given its relatively benign side-effect profile compared to these agents.


  • Relaxation techniques are generally considered safe for healthy people. However, occasionally, people report negative experiences such as increased anxiety, intrusive thoughts, or fear of losing control. There have been rare reports that certain relaxation techniques might cause or worsen symptoms in people with epilepsy or certain psychiatric conditions, or with a history of abuse or trauma.
  • Side effects of melatonin are uncommon but can include drowsiness, headache, dizziness, or nausea. There have been no reports of significant side effects of melatonin in children. In one study, researchers noted that melatonin supplements may worsen mood in people with dementia. According to AASM clinical guidelines, melatonin should not be used by older people with dementia.

Reducing Stress

Myth: The herb passionflower can reduce stress and improve overall health.

Fact: There are very few clinical trials of passionflower and therefore insufficient evidence to determine whether passionflower is efficacious for any condition.

However, there is some scientific evidence to date that suggests mindfulness meditation—a mind and body practice which cultivates abilities to maintain focused and clear attention, and develop increased awareness of the present—may help reduce symptoms of stress, including anxiety and depression. Results from a small body of research suggest that yoga may also affect stress and anxiety symptoms.

What Does the Research Show?

Three systematic reviews, including two with meta-analyses, have been published. The studies included have been small, preliminary studies, mostly randomized clinical trials. Overall, results from these studies suggest that mindfulness meditation has a small-to-moderate effect on various outcome measures, such as anxiety, depression, and pain. As trials were small and not designed to determine efficacy, further large-scale studies would be needed to better assess effects on mindfulness meditation for relieving stress. 

The evidence base on efficacy of yoga for stress consists of several randomized, nonrandomized, and observational studies; systematic reviews; and meta-analyses.

  • Mindfulness Meditation
    • 2014 systematic review and meta-analysis of 47 trials in 3,515 participants found that mindfulness meditation programs had moderate evidence of improved anxiety, depression, and pain, and low evidence of improved stress/distress and mental health-related quality of life. No evidence of benefit was found on meditation for health-related behaviors affected by stress, such as substance abuse and sleep.
    • 2014 systematic review of 17 studies on mindfulness-based stress reduction as a stress management intervention for healthy individuals found benefits in psychological or physiological outcomes related to anxiety and/or stress; however, many of the studies included in the review had methodological limitations and small sample sizes.
    • 2012 systematic review and meta-analysis of 36 randomized controlled trials examining the efficacy of meditation for anxiety symptoms found that 25 of the studies reviewed reported better outcomes in the meditation group compared to the control.
  • Yoga
    • 2015 systematic review of 25 randomized controlled trials provides preliminary evidence to suggest that yoga practice leads to better regulation of the sympathetic nervous system and hypothalamic-pituitary-adrenal system, as well as a decrease in depressive and anxious symptoms in a range of populations.
    • 2014 systematic review of 17 small studies examining the combination of mindfulness-based stress reduction and yoga in an 8-week training program found positive changes in psychological or physiological outcomes related to anxiety and/or stress. Further large-scale trials would be necessary to assess whether the combined intervention can reliably improve anxiety and/or stress.


  • Meditation is considered safe for healthy people. There have been rare reports that meditation could cause or worsen symptoms in people who have certain psychiatric problems, but this question has not been fully researched.
  • Overall, clinical trial data suggest yoga as taught and practiced in these research studies under the guidance of skilled teacher has a low rate of minor side effects. It is not uncommon for practitioners to have some minor, transient discomfort, as with most physical activity programs. However, injuries from yoga, some of them serious, have been reported in the popular press. People with health conditions should work with an experienced yoga instructor who can help modify or avoid some yoga poses to prevent side effects.

Preventing Colds

Myth: A daily dose of a vitamin C supplement will prevent the onset of the common cold.

Fact: Several reviews have concluded that prophylactic vitamin C does not reduce the incidence of colds in the general population, but may be useful in reducing incidence of colds for people exposed to brief periods of severe physical exercise (e.g., marathon runners, skiers, and soldiers training in subarctic conditions).

What Does the Research Show?

The available evidence on efficacy of vitamin C for treating or preventing the common cold consists of several randomized controlled trials and a few reviews, including a Cochrane review.

  • 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.

  • 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.


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

Reducing Cholesterol Levels or Other Known Cardiovascular Disease Risk Factors

Myth: Taking garlic supplements will prevent heart disease.

Fact: There is no evidence that garlic supplements prevent heart disease. Evidence is mixed about whether garlic supplements lower cholesterol levels or or change other known cardiovascular disease risk factors.

What Does the Research Show?

The available evidence on efficacy of garlic supplementation for disease prevention consists of mostly small, preliminary, or low-quality studies. Few clinical trials have been conducted to examine the potential anticarcinogenic effects of garlic.

  • 2012 Cochrane review (link is external) of two trials concluded that there is insufficient evidence to determine if garlic provides a therapeutic advantage over placebo in terms of reducing the risk of mortality and cardiovascular morbidity in hypertensive patients.
  • Findings from a 2013 meta-analysis of 39 small randomized controlled trials suggest garlic may reduce total serum cholesterol and low-density lipoprotein cholesterol in individuals with elevated total cholesterol levels, provided garlic is used for longer than 2 months. Minimal changes in high-density lipoprotein cholesterol levels and triglycerides were observed.

  • 2016 review of several small trials suggested that garlic may have favorable effects on some risk factors for cardiovascular disease, but whether such changes in risk factors alters progression of cardiovascular disease has not been fully studied. 


  • Garlic is generally safe for most people, with gastrointestinal discomfort as the most reported adverse effect.
  • Taking garlic supplements may increase bleeding risk, and people taking an anticoagulant should use caution.
  • Garlic has been found to interfere with the effectiveness of some drugs, including saquinavir, an antiretroviral drug used to treat HIV infection.

Preventing Cognitive Decline

Myth: Turmeric (curcumin) and Ginkgo biloba supplements can prevent the onset of dementia and Alzheimer’s disease in people.

Fact: Although there is some evidence in laboratory studies that curcumin may affect brain function and the development of dementia, these results have not been demonstrated in clinical trials. In several large clinical trials, Ginkgo biloba has been shown to be ineffective in reducing either the overall incidence rate of dementia or Alzheimer’s disease incidence.

What Does the Research Show?

Data on the use of turmeric (curcumin) for prevention of disease consists of mainly laboratory studies; clinical research is limited. In spite of its long history of use for a variety of disorders, there is insufficient evidence to support the use of turmeric supplementation.

The available evidence on efficacy of Ginkgo biloba for preventing dementia or cognitive decline consists of many randomized controlled trials, including a large, long-term study, which enrolled more than 3,000 older adults, as well as systematic reviews and meta-analyses.

  • Turmeric (curcumin). A 2016 review found that in vitro studies have shown that ß-amyloid metabolism is altered by curcumin, and animal studies suggest that curcumin may influence brain function and the development of dementia; however, clinical studies of curcumin to date have demonstrated only limited effects. A 2014 review found similar results, concluding that curcumin could potentially be studied in early-phase studies to determine whether it could be studied in Alzheimer’s disease.
  • Ginkgo biloba. The 2008 Ginkgo Evaluation Memory Study (GEMS), a randomized double-blind, placebo-controlled trial involving 3,069 participants aged 75 years or older with normal cognition at study entry found that Ginkgo biloba (120 mg twice a day) was not effective in reducing either the overall incidence rate of dementia or Alzheimer’s disease incidence. A 2012 randomized controlled trial involving 2,854 participants aged 70 years or older found that long-term use of Ginkgo biloba (120 mg twice a day over the course of 5 years) did not reduce the risk of progression to Alzheimer’s disease compared with placebo. A 2012 meta-analysis of 28 randomized controlled trials involving a total of 2,576 participants analyzed data on memory, executive function, and attention found that Ginkgo biloba had no ascertainable positive effects on a range of cognitive functions in healthy individuals.


  • Turmeric is generally considered safe when taken orally or applied to the skin. High doses or long-term use of turmeric may cause gastrointestinal problems. In 2016, the U.S. Food and Drug Administration recalled some brands of turmeric due to heavy lead contamination.
  • In general, ginkgo appears to be safe for most people when taken orally in moderate doses. Common side effects include headache, gastrointestinal discomfort, and allergic skin reactions. Eating fresh or roasted ginkgo seeds can be toxic.


  • Auger RR, Burgess HJ, Emens JS, et al. Clinical practice guideline for the treatment of intrinsic circadian rhythm sleep-wake disorders: advanced sleep-wake phase disorder (ASWPD), delayed sleep-wake phase disorder (DSWPD), non-24-hour sleep-wake rhythm disorder (N24SWD), and irregular sleep-wake rhythm disorder (ISWRD). An update for 2015. Journal of Clinical Sleep Medicine. 2015;11(10):1199–1236.
  • Barrett B, Hayney MS, Muller D, et al. Meditation or exercise for preventing acute respiratory infection: a randomized controlled trial. Ann Fam Med. 2012;10(4):337–346.
  • Chen KW, Berger CC, Manheimer E, et al. Meditative therapies for reducing anxiety: a systematic review and meta-analysis of randomized controlled trials. Depress Anxiety. 2012;29(7):545–562.
  • DeKosky ST, Williamson JD, Fitzpatrick AL, et al. Ginkgo biloba for prevention of dementia: a randomized controlled trial. JAMA. 2008;300(19):2253–2262.
  • Ferraciolo-Oda E, Qawasmi A, Bloch MH. Meta-analysis: melatonin for the treatment of primary sleep disorders. PLoS One. 2013;8(5):e63773.
  • Goozee KG, Shah TM, Sohrabi HR, et al. Examining the potential clinical value of curcumin in the prevention and diagnosis of Alzheimer’s disease. Br J Nutr. 2016;115(3):449–465.
  • Goyal M, Singh S, Sibinga EM, et al. Meditation programs for psychological stress and well-being: a systematic review and meta-analysis. JAMA Internal Medicine. 2014; 174(3): 357–368.
  • Herxheimer A, Petrie KJ. Melatonin for the prevention and treatment of jet lag. Cochrane Database of Systematic Reviews. 2002;2:CD001520.
  • Hölzel BK, Carmody J, Evans KC, et al. Stress reduction correlates with structural changes in the amygdala. Soc Cogn Affect Neurosci. 2010;5(1):11–17.
  • Laws KR, Sweetnam H, Kondel TK. Is Ginkgo biloba a cognitive enhancer in health individuals? A meta-analysis. Hum Psychopharmacol. 2012;27(6):527–533.
  • Mechanisms of action of phytochemicals from medicinal herbs in the treatment of Alzheimer’s disease. Planta Med. 2014;80(15):1249–1258.
  • Morganthaler T, Lee-Chiong T, Alessi C, et al. Practice parameters for the clinical evaluation and treatment of circadian rhythm sleep disorders. An American Academy of Sleep Medicine report. Sleep. 2007;30(11):1445–1459.
  • Morgenthaler T, Kramer M, Alessi C, et al. Practice parameters for the psychological and behavioral treatment of insomnia: an update. An American Academy of Sleep Medicine report. Sleep. 2006;29(11):1415–1419.
  • Morin CM, Bootzin RR, Buysse DJ, et al. Psychological and behavioral treatment of insomnia: update of the recent evidence (1998-2004). Sleep. 2006;29(11):1398–1414.
  • Obasi CN, Brown R, Ewers T, et al. Advantage of meditation over exercise in reducing cold and flu illness is related to improved function and quality of life. Influenza Other Respir Viruses. 2013;7(6):938–944.
  • Pascoe MC, Bauer IE. A systematic review of randomised control trials on the effects of yoga on stress measures and mood. J Psychiatr Res. 2015;68:270–282.
  • Ried K, Toben C, Fakler P. Effect of garlic on serum lipids: an updated meta-analysis. Nutr Rev. 2013;71(5):282–299.
  • Sharma M, Rush SE. Mindfulness-based stress reduction as a stress management intervention for healthy individuals: a systematic review. J Evid Based Complementary Altern Med. 2014;19(4):271–286.
  • Stabler SN, Tejani AM, Huynh F, et al. Garlic for the prevention of cardiovascular morbidity and mortality in hypertensive patients. Cochrane Database of Systematic Reviews. 2012;8:CD007653.
  • van der Kolk BA, Stone L, West J, et al. Yoga as an adjunctive treatment for posttraumatic stress disorder: a randomized controlled trial. J Clin Psychiatry. 2015;75(6):e559–565.
  • Varshney R, Budiff MJ. Garlic and heart disease. J Nutr. 2016;146(2):416S–421S.
  • Vellas B, Coley N, Ousset PJ, et al. Long-term use of standardised Ginkgo biloba extract for the prevention of Alzheimer’s disease (GuidAge): a randomised placebo-controlled trial. Lancet Neurol. 2012;11(10):851–859.

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