What is it? Overview Usage Side Effects and Warnings

Garlic Side Effects and Warnings

Written by FoundHealth, ColleenO.

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Safety Issues

As a commonly used food, garlic is on the FDA's GRAS (generally recognized as safe) list. Rats have been fed gigantic doses of aged garlic (2,000 mg per kilogram body weight) for 6 months without any signs of negative effects. 1 Long-term treatment with standardized garlic powder at a dose equivalent to three times the usual dose, along with fish oil, produced no toxic effects in rats. 2 The only common side effect of garlic is unpleasant breath odor. Even "odorless garlic" produces an offensive smell in up to 50% of those who use it. 3 Other side effects occur only rarely. For example, a study that followed 1,997 people who were given a normal dose of deodorized garlic daily over a 16-week period showed a 6% incidence of nausea, a 1.3% incidence of dizziness on standing (perhaps a sign of low blood pressure), and a 1.1% incidence of allergic reactions. 4 There were also a few reports of bloating, headaches, sweating, and dizziness.

When raw garlic is taken in excessive doses, it can cause numerous symptoms, such as stomach upset, heartburn, nausea, vomiting, diarrhea, flatulence, facial flushing, rapid pulse, and insomnia.

Topical garlic can cause skin irritation, blistering, and even third-degree burns, so be very careful about applying garlic directly to the skin. 5 Since garlic might "thin" the blood, it is probably imprudent to take garlic pills immediately prior to or after surgery or labor and delivery, because of the risk of excessive bleeding. 6 Similarly, garlic should not be combined with blood-thinning drugs, such as warfarin (Coumadin), heparin , aspirin , clopidogrel (Plavix), ticlopidine (Ticlid), or pentoxifylline (Trental). In addition, garlic could conceivably interact with natural products with blood-thinning properties, such as ginkgo , policosanol , or high-dose vitamin E . However, a placebo-controlled study found that actual raw garlic consumed in food at the fairly high dose of 4.2 mg once daily did not impair platelet function. In addition, volunteers who continued to consume the dietary garlic for a week did not show any change in their normal platelet function. 7 Garlic may also combine poorly with certain HIV medications. Two people with HIV experienced severe gastrointestinal toxicity from the HIV drug ritonavir after taking garlic supplements. 8 Garlic might also reduce the effectiveness of some drugs used for HIV. 9 Garlic is presumed to be safe for pregnant women (except just before and immediately after delivery) and nursing mothers, although this has not been proven.

Interactions You Should Know About

If you are taking:

  • Ginkgo , policosanol , or high-dose vitamin E : Taking garlic at the same time might conceivably cause a risk of bleeding problems.
  • Medications for HIV : Do not use garlic.


  1. Sumiyoshi H, Kanezawa A, Masamoto K, et al. Chronic toxicity test of garlic extract in rats [in Japanese; English abstract]. J Toxicol Sci. 1984;9:61-75.
  2. Morcos NC, Camilo K. Acute and chronic toxicity study of fish oil and garlic combination. Int J Vitam Nutr Res. 71(5):306-12.
  3. Schulz V, Hansel R, Tyler VE. Rational Phytotherapy: A Physicians' Guide to Herbal Medicine. 3rd ed. Berlin, Germany: Springer-Verlag; 1998:121.
  4. Schulz V, Hansel R, Tyler VE. Rational Phytotherapy: A Physicians' Guide to Herbal Medicine. 3rd ed. Berlin, Germany: Springer-Verlag; 1998:121.
  5. Garty BZ. Garlic burns. Pediatrics. 91(3):658-9.
  6. Burnham BE. Garlic as a possible risk for postoperative bleeding. Plast Reconstr Surg. 95(1):213.
  7. Scharbert G, Kalb ML, Duris M, et al. Garlic at dietary doses does not impair platelet function. Anesth Analg. 2007;105:1214-1218
  8. Piscitelli SC. Use of complementary medicines by patients with HIV: Full sail into uncharted waters. Medscape HIV/AIDS. 2000;6.
  9. Piscitelli SC, Burstein AH, Welden N, Gallicano KD, Falloon J. The effect of garlic supplements on the pharmacokinetics of saquinavir. Clin Infect Dis. 34(2):234-8.

1 Comment

Posted 11 years ago

i wonder if garlic is overrated china has biggest consumption of garlic per head and has biggest stomach cancer rate per head according to world health organisation they said this is due to eating lot of fungal my own experience with garlic i have developed these spells dream like state for few seconds also before asleep smallest squeak in the room it will send a pain trough my body at the speed of light and many times i feel like my whole body nervous system will switch off.been trough all neurological tests all good. mention it to neurolog he said never heard or know about anything like it. stop eating garlic all gone. and one more there is no cure or operation for wrong diet.candida is the culprit of almost any disease including diabetes and cancer

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mat.j.collins 11 years ago

Hi borrisziv. great point, garlic is not a cure-all and other diet behaviors are necessary in achieving optimal health. Cautions related to garlic are important, such as potentially ingesting too much and having allergenic responses to it. Thanks for sharing your thoughts and experiences.

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