Garlic has antiplatelet properties, and can increase the risk of bleeding when used together with drugs with antiplatelet and anticoagulant effects, such as aspirin, clopidogrel, ticlopidine, dipyridamole, heparins, and warfarin (3). Increased international normalized ratio (INR), has been reported when garlic was added to warfarin (106). Garlic supplements that contain allicin can induce the cytochrome P450 3A4 (CYP 3A4) isoenzyme and can result in clinically important decreases in concentrations of drugs metabolized by this enzyme. This interaction was proven with saquinavir (107). However, a garlic preparation containing alliin and alliinase (which formed one-half the amount of allicin stated on the label) did not significantly inhibit CYP 3A4, which was proven by a lack of interaction with the drug alprazolam (108). It is not known whether the differences in the preparations and dose of allicin or some other factor in the metabolism of saquinavir, such as P-glycoprotein, are responsible for this effect. Until more data is available, it would be prudent to avoid or use caution when allicin is used together with some drugs metabolized by CYP 3A4, including protease inhibitors, cyclosporine, ketoconazole, itraconazole, glucocorticoids, oral contraceptives, verapamil, diltiazem, lovastatin, simvastatin, and atorvastatin.

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