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Hypertension and Garlic

Written by ColleenO, FoundHealth.

Garlic, both as a food and as a supplement, is often recommended as a treatment for heart-related conditions, such as heart attack. Some evidence suggests that it might have a modest effect on lowering blood pressure.

Effect of Garlic on Hypertension

Garlic may help treat high blood pressure by, among other things, acting as a blood thinner.

Read more details about Garlic.

Research Evidence on Garlic

Numerous studies have found weak evidence that garlic lowers blood pressure slightly, perhaps in the neighborhood of 5% to 10% more than placebo.65-67,120 It remains unclear whether garlic supplements can help patients with high blood pressure safely eliminate or avoid antihypertensive medications.

One study followed 47 subjects with an average starting blood pressure of 171/101.68 Over a period of 12 weeks, half were treated with 600 mg of garlic powder daily standardized to 1.3% alliin, while the other half were given placebo. The results showed a statistically significant drop of 11% in the systolic blood pressure and 13% in the diastolic pressure. In comparison, blood pressure fell in the placebo group by 5% and 4%, respectively. (Note: the average starting blood pressure of the placebo and the treated groups were quite different, making comparisons problematic.)

How to Use Garlic

Garlic can be taken in many forms: fresh; cooked; aged, or preserved; as a powder, perhaps in capsules; and in an extract form, also in capsules.

The garlic used in one of the studies here was a powdered garlic supplement (1.3% alliin), taken at 600 mg/day. (A typical dosage of garlic is 900 mg daily of a garlic powder extract standardized to contain 1.3% alliin, providing about 12,000 mcg of alliin daily, or 4-5 mg of “allicin potential.” Alliin-free aged garlic is taken at a dose of 1 to 7.2 g daily.)

Garlic supplements of various forms can be found in health food stores and drug stores. Products vary in quality, primarily because some of garlic's active components can deteriorate as a result of processing.

Alliin is a relatively odorless substance found in garlic. When garlic is crushed or cut, an enzyme called allinase is brought in contact with alliin, turning it into allicin. Allicin is responsible for much of the typical odor of garlic. It is very active chemically and probably helps the garlic bulb defend itself from attack by insects and other threats. However, allicin is unstable, and soon breaks down into a variety of other substances. When garlic is ground up and encapsulated, the effect is similar to cutting the bulb: Alliin contacts allinase, yielding allicin, which then breaks down. Unless something is done to prevent this process, garlic powder won't have any alliin or allicin left by the time it is purchased.

Some garlic producers believe that alliin and allicin are not essential for garlic's effectiveness and do not worry about this. Aged garlic, for example, has very little of either compound. But other manufacturers believe that allicin is the primary active ingredient in garlic. Because allicin is an unstable chemical, these manufacturers are faced with a challenge.

One solution might be to chemically stabilize allicin so that it doesn’t break down. However, allicin has a strong garlic smell, and a relatively odorless product is preferable. Many manufacturers of garlic powder products seek to stabilize the alliin in the product, and to do so in such a way that the alliin converts to allicin after it is consumed. How well their methods work remain a matter of controversy.

Note: Do not confuse essential oil of garlic with garlic oils. The term "garlic oil" refers to garlic extracted by means of oil. Garlic essential oil is the pure oily component of the herb, and, like other essential oils, it is not intended for oral consumption and is potentially toxic.

Types of Professionals That Would Be Involved with This Treatment

  • Naturopathic doctor
  • Herbalist
  • Nutritionist or dietitian
  • Integrative MD

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. Silagy CA, Neil HA. A meta-analysis of the effect of garlic on blood pressure. J Hypertens. 1994;12:463-468.
  2. Schulz V, Hansel R, Tyler VE. Rational Phytotherapy: A Physicians' Guide to Herbal Medicine. 3rd ed. Berlin Germany: Springer-Verlag; 1998:119.
  3. Auer W, Eiber A, Hertkorn E, et al. Hypertension and hyperlipidaemia: garlic helps in mild cases. Br J Clin Pract Suppl. 1990;69:3-6.
  4. Auer W, Eiber A, Hertkorn E, et al. Hypertension and hyperlipidaemia: garlic helps in mild cases. Br J Clin Pract Suppl. 1990;69:3-6.
  5. Ried K, Frank OR, Stocks NP, et al. Effect of garlic on blood pressure: a systematic review and meta-analysis. BMC Cardiovasc Disord. 2008 Jun 16.

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