The FDA is notifying health-care professionals about potential dangerous drug interactions between the hepatitis C virus (HCV) protease inhibitor boceprevir (Victrelis, Merck) and certain anti-HIV drugs that contain ritonavir (Norvir, Abbott).

Using these medications together can potentially reduce their effectiveness. However, the agency cautioned that patients already taking boceprevir and an HIV treatment regimen that contains ritonavir should not stop treatment without consulting their health-care provider.

Clinicians should monitor patients who are taking these medications to ensure that hepatitis C treatment response continues and that virologic rebound of either hepatitis C or HIV does not occur.

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The recommendations come following an FDA drug safety communication in February that warned about potential drug interactions between boceprevir and three ritonavir-boosted HIV drugs: atazanavir (Reyataz), lopinavir (Kaletra) and darunavir (Prezista).

Boceprevir is a hepatitis C virus (HCV) protease inhibitor that was approved last year to treat chronic hepatitis C infection in adults. Ritonavir is a weaker HIV protease inhibitor used to “boost” other HIV protease inhibitors to increase their levels in the blood and make them more effective.

The drug label for boceprevir will be updated to include information about these drug interactions. Another newly approved HCV drug, telaprevir (Incivek), already includes cautions about drug interactions with HIV medications. Neither drug is approved for patients coinfected with HIV and HCV.

This article originally appeared on MPR