Contra-Indications of Garlic

Garlic is on the FDA’s GRAS list. However, garlic is usually cooked when taken as food. During the heating process much of the chemical constituents of garlic such as alliin are destroyed. Typical standardized garlic products contact substances (such as alliin) not found the food garlic, making them more similar to raw garlic. Raw garlic taken in excessive doses can reportedly 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.

Rats fed up to 2000 mg/kg of aged garlic extract for six months, showed no significant toxicity. Genotoxicity and mutagenicity studies have been negative for aged and fresh garlic. In a chronic toxicity study in rats, long-term treatment (12 months) with standardized garlic powder at a dose equivalent to three times the usual dose, produced no toxic effects in rats. (Fish oil was administered simultaneously.)

In studies, garlic powder has not been associated with serious side effects. For example, an observational study followed 1,997 patients given 300 mg. of garlic powder three times daily over a 16 week period, and found a six percent incidence of nausea, a 1.3 percent incidence of orthostasis, and a 1.1 percent incidence of allergic reactions attributable to garlic. There were also a few reports of bloating, headache, sweating and dizziness.

The most common problem with garlic is the odor. The use of so-called odorless products can alleviate, though not entirely eliminate, this problem. One study suggests that therapeutic levels of odorless garlic produces an offensive garlic smell in 50 percent of patients, perhaps because allicin formed in the digestive tract. This reduces some patient compliance.

For theoretical reasons, garlic is not recommended for patients with brittle diabetes garlic might possess a hypolgycemic effect), semphigus (activated by sulphus-containing compounds), organ transplants (possible activation of immune rejection), or acute rhumatoid arthritis (possible increase in autoimmunity).

At least one case report associated garlic supplement use with a spontaneous spinal epidural hematoma. Based on reports of increased bleeding following surgery, the European Scientific Cooperation on Phytotherapy recommends against using garlic before, during or immediately after surgical procedures. Maximum safe dosages in individuals with severe hepatic or renal disease are not known.

Cooked garlic is presumed to be safe in pregnancy and lactation based on its extensive food use. However, maximums safe dosage of standardized garlic extracts in these groups, as well as in young children and individuals with severe live or renal disease, has not been established. There have not been any studies on teratogenicity or embryo toxicity of any form of garlic.

Given the report of increased bleeding following a surgical procedure mentioned above, it might also be wise to avoid garlic supplementation during the period before and after labor and delivery.

Because garlic appears to possess antithrombotic activity, it should be used with caution in patients on anticoagulants. One report has indicated an increase in bleeding time in two patients taking both warfarin and garlic supplements. It is also conceivable that garlic could interact with other natural substances with anticoagulant effects, such as gingko, policosanol (also used for hypolipidemia), and high-dose vitamin E.

Garlic has been found to reduce plasma concentrations of saquinavir. In addition, two individuals with HIV experienced severe gastrointestinal toxicity from ritonavir after taking garlic supplements. 10