Protease inhibitors and sildenafil (Viagra) should not be combined.

The European Agency for the Evaluation of Medicinal Products (EMEA) issued a warning in April 1999, about potential adverse effects when the impotence drug sildenafil (Viagra) is taken with protease inhibitors. In particular, sildenafil and Ritonavir in combination results in significantly increased plasma levels of sildenafil for up to 24 hours after the drug is taken. There are similar reactions with other drugs, including Saquinavir, Erythromycin, and Itraconazole. The EMEA recommends that anyone taking protease inhibitors refrain from sildenafil, and that anyone taking sildenafil not exceed 25 mg in a 48-hour period.