Garlic is one of the most extensively studied herbal medications available. It has been thought to possess antimicrobial, anti-inflammatory, antifungal, antiprotozoal, antioxidant, antineoplastic, and antithrombotic properties that make its use as a general tonic attractive. Recent research has primarily focused on the use of garlic for the treatment of cardiovascular diseases and risk factors. Although allicin has been proposed to be the active ingredient within garlic (54,55), its poor bioavailability limits its direct effects (56). The overall effectiveness of garlic probably revolves around the synergistic effect of various organosulfur compounds produced from the metabolism of g-glutamylcysteines found within intact garlic bulbs (56). These compounds may act to inhibit cholesterol synthesis (54,57), alter platelet function (58), and cause smooth-muscle relaxation (59).
A number of clinical trials have been performed using a variety of garlic compounds. These trials have generated conflicting results. A recent meta-analysis has suggested that garlic supplementation may decrease total cholesterol, triglycerides, and LDL levels modestly, but only in the short term (60). Analysis of studies longer than 24 weeks failed to reveal significant effects (60). High-density lipoprotein levels do not seem to be affected by garlic administration (60). When used in the short term, garlic seems to improve cholesterol levels by 4-6% (61). When compared to the 17-32% sustained decrease in cholesterol levels seen with statin drugs (62,63), the use of garlic cannot be endorsed as a viable alternative for the treatment of hypercholesterolemia. Additionally, its use for the treatment ofhypertension, diabetes, or peripheral vascular disease is not supported (60,64).
Garlic toxicity is usually mild and consists of gastrointestinal upset and breath and body odor. Given its potential effects on platelet function, an increased risk of bleeding may be present. One case of a spontaneous spinal epidural hematoma has been reported with excessive garlic consumption (65). Caution should be taken with concomitant use of garlic with anticoagulants. An increase in the International Normalized Ratio of a patient taking garlic and warfarin has been reported (66). Recent concern over an interaction between garlic and protease inhibitors used for the treatment of HIV has emerged. Certain prepartations of garlic have been shown to decrease the peak levels of saquinavir by 54% (67).
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