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Case 1: Drug Interactions with Antiretroviral Medications

You answered:

B Ritonavir can significantly increase levels of sildenafil.

This answer is correct. Sildenafil is a substrate of CYP3A4. Ritonavir and all currently available protease inhibitors inhibit the CYP3A4 isozyme to some degree. Inhibiting the CYP3A4 enzyme results in slower elimination of sildenafil. As a result, sildenafil should be used with caution when combined with a protease inhibitor, with a starting dose of 25 mg of sildenafil recommended.


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