Garlic supplements may lower blood concentrations of the protease inhibitor Invirase (saquinavir) and should not be used at the same time if an Invirase-boosting dose of Norvir (ritonavir) is not also being taken. The new recommendation, announced by the U.S. Food and Drug Administration, comes as part of a larger revision to the package insert for Invirase.

Scientists have known about the potential drug interaction since 2001, when a study by the National Institutes of Health (NIH) found that garlic capsules reduced blood levels of saquinavir, in the absence of Norvir, by almost 50 percent. Garlic can cause the body to produce more of an enzyme called CYP 3A4, which is needed to break down saquinavir and many other ARV drugs, thus causing those drugs to be eliminated from the body more rapidly than is desired.

The latest revision of Invirase’s package insert now says that the combination of unboosted Invirase and garlic capsules is “not recommended.” Provided that low-dose Norvir booster is used, which is currently recommended for anyone taking Invirase, supplementing with garlic capsules is permitted.

Further revisions to the package insert include drug interaction warnings and dosing recommendations for Invirase and several other drugs, including methadone, Prilosec (omeprazole), the heart drug digoxin and the protease inhibitor Aptivus (tipranivir).

Although garlic can potentially affect blood levels of other ARV drugs, the FDA is issuing a warning only for uboosted Invirase at this time. The recommendation applies only to the use of garlic supplements, as these contain much higher concentrations of the herb than the raw product.