Common Names
- Nectar of the gods
- Camphor of the poor
- Da-suan
- La-suan
- Stinking rose
For Patients & Caregivers
How It Works
Evidence on whether garlic can help lower cholesterol levels or blood pressure are mixed. It is associated with a decreased risk for some cancers, but there is no evidence that it can treat cancer.
Derived from the bulb or clove of the plant, garlic is used as a spice worldwide. It is also used traditionally for a variety of conditions such as maintaining circulatory function, to treat infections, and to boost the immune system. Raw garlic contains compounds called alliin and allicin. In lab studies, these compounds and their breakdown products killed bacteria, reduced platelets and lipids in the blood, and slowed clotting.
Studies on whether garlic can reduce cholesterol or lower blood pressure are mixed, but other data suggest it may help reduce some cardiovascular risk factors, stimulate the immune system, or protect against some cancers.
Processing may reduce the active compounds found in garlic products. For example, garlic powder and garlic essential oil do not contain allicin or ajoene, the compounds believed to have cholesterol-reducing and blood-thinning properties.
Patients taking warfarin or other blood thinners should ask their doctor before taking garlic supplements.
Purported Uses
-
To treat cardiovascular disease
Clinical trials studying the effect of garlic on blood pressure and cholesterol levels have shown mixed results, but other analyses suggest it may help reduce some risk factors for cardiovascular disease. Patients should ask their doctor before taking garlic supplements. -
To prevent and treat cancer
Evidence on whether garlic can reduce the risk of gastric and colorectal cancers is mixed. Garlic has been associated with a decreased risk for some other types of cancer as well, but there is no evidence that it can treat cancer. -
To treat infections
Lab experiments and small studies in humans suggest that garlic stimulates the immune system and has antimicrobial activity, but it is unclear whether it can treat infections.
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 are taking protease inhibitors (darunavir/Prezista®, saquinavir/Fortovase®, Invirase®): In 2 cases of HIV patients who regularly consumed garlic, darunavir became ineffective. Other studies in humans found garlic significantly reduced blood levels of saquinavir and may therefore also reduce its 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.
- You are taking drugs that are substrates of cytochrome P450 2C9, 2C19, or 3A4: Garlic may increase the risk of side effects of these drugs.
- You are taking drugs that are substrates of P-glycoprotein: Garlic may reduce the activity of such drugs.
Side Effects
- Headache
- Fatigue
- Offensive odor, bad breath
- Stomach upset
- Diarrhea
- Changes in natural intestinal bacteria
- Sweating
- Low blood sugar
- Altered platelet function
- Increased bleeding risk
Case Reports (Oral)
- Bleeding, reduced blood clotting: Following excessive use of garlic supplements.
- Redetected levels of HIV: In 2 cases due to interactions between garlic and darunavir. The effectiveness of the medication returned some time after garlic intake was stopped.
- Chemical burns in the mouth lining: Following consumption of crushed garlic.
- Allergic reactions: Two cases, following ingestion of garlic.
- Liver toxicity: After a liver transplant patient was started on high-dose garlic supplementation to treat low levels of oxygen in the blood.
Case Reports (Topical)
- Chemical burns: Multiple cases from the topical use of garlic in attempts to treat warts, rash/itching, tooth pain, acne, and sore throat.
- Skin rash: Due to “garlic necklace” on infant’s neck to treat nasal congestion
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. Processing can have a substantial effect on the chemical content of garlic because the volatile oil components are heat-sensitive and certain enzymes are acid-labile. The best measure of 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 evaluated a variety of the proposed claims.
Various placebo-controlled trials on cholesterol-lowering effects of garlic yielded mixed results (2) (3) (4) (5) (6) (7) (31). Data on whether garlic preparations may lower blood pressure are also mixed (45) (53) (55), but other meta-analyses suggest reductions in cardiovascular risk factors due to antihyperlipidemic effects, reduced inflammatory biomarkers, and improved glucose levels (50) (56) (57) (58) (59). In other studies, aged garlic extract did not improve metabolic parameters in diabetic patients at high cardiovascular risk (60), but reduced low-attenuation plaque volume (61). Garlic extract also improved endothelial biomarkers associated with cardiovascular risk in obese individuals (62). Data also suggest immunostimulatory effects (53) and benefits in patients with hepatopulmonary syndrome (32), but more data are needed to determine if garlic is effective against the common cold (33) and vaginal candidiasis (47).
A number of studies have evaluated whether garlic products can have protective effects against various cancers. In a large randomized intervention trial, long-term garlic supplementation was associated with reduced risk of gastric cancer mortality but not incidence (63), although conflicting data did not find evidence for either (43). Whether garlic protects against Helicobacter pylori infection is also unclear (64). Meta-analyses are also mixed on whether garlic intake reduces colorectal cancer risk (65) (66). Preliminary studies suggest aged garlic extract may reduce number and size of subsequent colorectal adenomas in patients with a history of adenomas (15), and improve natural killer cell number and activity but not quality of life in patients with advanced cancers (14). Inverse associations have been identified between garlic consumption and risk of endometrial (13), prostate (12), and other cancers (9). 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).
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 has antibacterial and antimicrobial activity, but is highly odoriferous and unstable. It also has antiplatelet 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
Adverse Reactions
Headache, fatigue, altered platelet function with potential for bleeding, offensive odor, GI upset, diarrhea, sweating, changes in the intestinal flora, hypoglycemia (16).
Case Reports (Oral)
- Anaphylaxis (38) (54)
- Perioperative bleeding: Following excessive garlic intake (46) (70).
- Prolonged bleeding, spinal epidural hematoma, platelet dysfunction: Following excessive use of garlic (29).
- Renal hematoma following lithotripsy: In a 51-year-old man, due to excessive odorless garlic ingestion. Treated with antibiotics and fluids; remission occurred after 5 months (35).
- Viral rebound in HIV patients: Two cases occurred due to significant interactions between garlic and darunavir that resulted in sub-therapeutic DRV plasma concentrations (71).
- Hepatotoxicity: After a liver transplant patient was started on high-dose garlic supplementation for presenting hypoxemia and hepatopulmonary syndrome (72).
- Chemical burn of oral mucosa: Following consumption of crushed garlic (36).
Case Reports (Topical)
Herb-Drug Interactions
- Warfarin: Anticoagulant activity may be enhanced due to increased fibrinolytic activity and diminished human platelet aggregation (17) (49).
- Protease inhibitors (darunavir, saquinavir): In 2 cases of HIV-infected patients, clinically significant interactions between garlic and darunavir that resulted in sub-therapeutic DRV plasma concentrations and viral rebound (71). In one case the patient ingested 15 garlic cloves per week, while in the other the patient could not quantify the amount, but said it was significant. In both patients, drug effectiveness eventually returned with the cessation of garlic. In healthy volunteers, garlic supplements with the rough equivalent of two 4-g cloves of garlic daily significantly decreased mean serum concentration levels, peak levels by 54%, and trough levels by 49%. These reductions in levels can cause therapeutic failure (18).
- Insulin: Dose of insulin may require adjustment due to hypoglycemic effects of garlic exhibited in animal studies (39).
- Cytochrome (CYP) P450 substrates: In vitro studies suggest that garlic products may inhibit CYP 2C9 and 2C19, and may interfere with the drugs metabolized by these enzymes (41) (42). Studies on its effects on 3A4 are mixed (34) (41) (42).
- P-Glycoprotein substrates: In healthy volunteers, garlic extract induced P-glycoprotein and can interfere with the metabolism of certain drugs (34).