In an in-vivo human metabolism study enfuvirtide, at the recommended dose of 90 mg twice daily, did not inhibit the metabolism of substrates by CYP3A4 (dapsone), CYP2D6 (debrisoquine), CYP1A2 (caffeine), CYP2C19 (mephenytoin), and CYP2E1 (chlorzoxazone).