Common Names
- Nectar of the gods
- camphor of the poor
- da-suan
- la-suan
- stinking rose
For Patients & Caregivers
How It Works
Garlic was shown to lower blood cholesterol and triglyceride levels. It is associated with decreased risk of some cancers, but there is no evidence that it can treat cancer.
Raw garlic contains compounds called alliin and allicin. In laboratory studies, these compounds and their breakdown products have been found to kill bacteria directly, reduce the number of platelets in the blood and slow clotting, and reduce the level of lipids in the blood. Compounds called ajoenes are also responsible for garlic’s ability to prevent blood clots. Garlic may also reduce blood pressure. Garlic intake may protect against certain cancers possibly by decreasing tumor cell growth or stimulating the immune system.
Processing can have a significant effect on the amount of active compounds in garlic: Garlic powder and garlic essential oil do not contain allicin or ajoene, compounds believed to be responsible for garlic’s cholesterol-reducing and blood-thinning properties.
Because garlic has blood thinning property, patients taking warfarin or other blood thinners should ask their doctor before taking garlic supplements.
Purported Uses
-
To fight skin infections
Laboratory studies show that raw garlic has antibacterial, antiviral, antiparasitic, and antifungal activity. -
To prevent and treat cancer
A few clinical trials suggest that high garlic consumption may lower the risk of stomach and colorectal cancers. But a recent study did not find such benefit. -
To treat heart disease
Several clinical trials show that use of garlic supplements reduces cholesterol and triglyceride levels in the blood, which may reduce the risk of heart disease, but a handful of clinical trials contradict these findings. Clinical trials studying the effect of garlic on blood pressure and risk of heart disease have shown mixed results. -
As an antioxidant
No scientific evidence supports this use. -
To treat atherosclerosis
Several clinical trials show that use of garlic supplements reduces blood pressure as well as cholesterol and triglyceride blood levels, which are risk factors for developing atherosclerosis, but a handful of clinical trials contradict these findings. Clinical and laboratory studies support garlic’s antiplatelet activity, which may help prevent blood clots in patients with atherosclerosis. -
To lower high cholesterol
Several clinical trials show that use of garlic supplements reduces cholesterol and triglyceride levels in the blood, but some clinical trials contradict these findings. -
To lower high blood pressure
Clinical trials studying the effects of garlic on blood pressure have shown mixed results. -
To treat circulatory disorders
Clinical and laboratory studies support garlic’s antiplatelet activity, which may help prevent blood clots in patients with circulatory disorders.
Patient Warnings
Do Not Take If
- You are taking warfarin or other blood thinners (Garlic may increase the risk of bleeding or bruising).
- You take cyclosporine (Garlic can reduce its effectiveness and potentially cause transplant rejection).
- You take protease inhibitors such as saquinavir (Fortovase®, Invirase®) (Garlic can significantly reduce their levels in the blood and reduce their effectiveness).
- You are taking insulin (Dose adjustments may be required because of the occasional blood sugar-lowering effect of garlic supplements. Take with caution and consult your doctor).
- If you are taking drugs that are substrates of Cytochrome P450 2C9, 2C19, 3A4 (Garlic may increase the risk of side effects of these drugs).
- If you are taking drugs that are substrates of P-Glycoprotein (Garlic may reduce the activity of such drugs).
Side Effects
- Headache
- Fatigue
- Altered platelet function with potential for bleeding
- Offensive odor, bad breath
- Stomach upset
- Diarrhea
- Changes in the natural bacteria found in the intestines
- Sweating
- Low blood sugar
- Contact dermatitis (inflammation, redness of the skin) when used topically.
- Following excessive use of garlic supplements, one patient experienced prolonged bleeding time, diminished platelet blood clotting activity, and spinal epidural hematoma (collection of blood in the spinal canal).
- A 51-year-old man developed renal hematoma after extracorporeal shock-wave lithotripsy (SWL) that occurred due to excessive odorless garlic ingestion. He was treated with antibiotics and fluids. His hematoma went into remission after five months.
- Chemical burn of oral mucosa has been reported following consumption of crushed garlic.
- Two episodes of severe, near-fatal anaphylaxis were reported in a 52-year-old man following ingestion of garlic.
- Topical application of garlic to treat a facial wart resulted in a burn in a 23-year-old woman.
- Topical application of garlic and salt paste under occlusion caused a severe chemical burn in a 41-year-old man. His symptoms resolved after treatment and discontinuation of garlic use.
- Use of an external garlic poultice for toothache resulted in significant chemical burn to the face.
- An unusual garlic burn was reported on the neck of a patient, following application of crushed raw garlic to address symptoms of a sore throat.
For Healthcare Professionals
Clinical Summary
Derived from the bulb or clove of the plant, garlic is used as a spice and to treat hyperlipidemia, hypertension, atherosclerosis, cancer, and infections. Because processing can have a substantial effect on the chemical content of garlic (the volatile oil components are sensitive to heat and certain enzymes are acid-labile), the best measure of the total activity of garlic is its ability to produce allicin, which in turn, results in the formation of other active constituents (1). Several oral garlic formulations are available, and clinical studies have addressed a variety of the proposed claims.
Placebo-controlled trials on the cholesterol lowering effect of garlic yielded mixed results (2) (3) (4) (5) (6) (7) (31), but a systematic review showed that garlic is effective in lowering total cholesterol and LDL cholesterol levels (37). Studies evaluating the antithrombotic effects repeatedly show modest reduction in platelet aggregation, but varying levels of fibrinolytic activity; and mixed effects with regard to reductions in blood glucose, blood pressure, or risk of cardiovascular disease (8). However, meta analyses show that there is consistent evidence to support use of garlic in lowering cardiovascular risk factors (50); and that garlic supplements have the potential to decrease blood pressure in hypertensive individuals, to regulate cholesterol levels, and have immunostimulatory effects (53). Garlic supplementation may also benefit patients with hepatopulmonary syndrome (32). More data are needed to determine if garlic is effective against common cold (33) and vaginal candidiasis (47). Whether garlic is effective in reducing the risk of mortality and cardiovascular morbidity in patients diagnosed with hypertension also remains inconclusive (45).
An analysis of several case-controlled studies in Europe suggests an inverse association between garlic consumption and risk of common cancers (9). High intake of garlic may be protective against gastric (10) and colorectal cancers (11); however, conflicting data indicate that long-term supplementation with garlic does not significantly reduce gastric cancer incidence nor mortality (43). Garlic intake was inversely associated with cancer of the prostate (12) and endometrium (13). In patients with advanced cancers, aged garlic extract (AGE) improved natural killer (NK) cell number and activity, but not quality of life (14). In patients with a history of adenomas, supplementation with AGE reduced both the number and size of subsequent colorectal adenomas (15). Garlic supplementation may also be associated with reduced risk of hematologic malignancies (40).
Because garlic is known to decrease platelet aggregation and potentially elevate International Normalized Ratio (INR) values, it should not be used with anticoagulants or in patients with platelet dysfunction (17).
Purported Uses
Mechanism of Action
The intact cells of garlic contain an odorless, sulfur-containing amino acid derivative known as alliin. When the cells are crushed, alliin comes into contact with the enzyme alliinase located in neighboring cells and is converted to allicin. Allicin is a potent antibiotic, but it is highly odoriferous and unstable. It is described as possessing antiplatelet, antibiotic, and antihyperlipidemic activities. Most authorities agree that the best measure of the total activity of garlic is its ability to produce allicin, which, in turn, results in the formation of other active constituents (1).
In patients with hyperlipidemia, garlic might lower cholesterol levels by acting as an HMG-CoA reductase inhibitor (21). For atherosclerosis, garlic is believed to reduce oxidative stress and low-density lipoprotein oxidation and have antithrombotic effects (22). It is also thought to reduce blood pressure by causing smooth muscle relaxation and vasodilation by activating the production of endothelium-derived relaxation factor (23).
Garlic may stimulate both humoral and cellular immunity, causing T-cell proliferation, restoring suppressed antibody responses (24), and stimulating macrophage cytotoxicity on tumor cells. It may increase selenium absorption with possible protection against tumorigenesis (25). In addition, garlic may protect against certain cancers by halting cell cycle progression and inducing apoptosis of cancer cells as well as by decreasing angiogenesis and influencing carcinogen metabolism (26) (27).
Warnings
Discontinue use of garlic at least 7 days prior to surgery.
(17)
Adverse Reactions
- Case Reports:
- Headache, fatigue, altered platelet function with potential for bleeding, offensive odor, GI upset, diarrhea, sweating, changes in the intestinal flora, hypoglycemia (16).
- Prolonged bleeding time with spinal epidural hematoma and platelet dysfunction has occurred following excessive usage of garlic (29).
- A 51-year-old man developed renal hematoma after extracorporeal shock-wave lithotripsy (SWL) that occurred due to excessive odorless garlic ingestion. He was treated with antibiotics and fluids. His hematoma went into remission after five months (35).
- Chemical burn of oral mucosa has been reported following consumption of crushed garlic (36).
- Anaphylaxis (38) (54)
- Topical application of garlic to treat a facial wart resulted in a burn in a 23-year-old woman (44).
- A 37-year-old woman who ate fresh garlic every day developed severe postoperative bleeding following hysterectomy (46).
- Topical application of garlic and salt paste under occlusion caused a severe chemical burn in a 41-year-old man. His symptoms resolved after treatment and discontinuation of garlic use (48).
- Use of an external garlic poultice for toothache resulted in significant chemical burn to the face (51).
- An unusual garlic burn was reported on the neck of a patient, following application of crushed raw garlic to address symptoms of a sore throat (52).
Herb-Drug Interactions
- Insulin: Dose of insulin may require adjustment due to hypoglycemic effects of garlic (39).
- Warfarin: Anticoagulant activity may be enhanced due to increased fibrinolytic activity and diminished human platelet aggregation (17) (49).
- Saquinavir (Fortovase, Invirase): Consuming garlic can significantly decrease serum concentration levels. Garlic can decrease peak levels by 54% and mean trough levels by 49%. These reductions in levels can cause therapeutic failure (18).
- Cytochrome (CYP) P450 substrates: Garlic can inhibit CYP 2C9, 2C19, 3A4 and may interfere with the drugs metabolized by these enzymes (41) (42).
- P-Glycoprotein substrates: Garlic induces P-glycoprotein and can interfere with the metabolism of certain drugs (34).