United Kingdom. Researchers consider that an interaction between sildenafil and HIV protease inhibitors is probable. HIV protease inhibitors inhibit the main cytochrome P450 isoenzyme by which sildenafil is metabolized. Concomitant administration of the drugs could lead to potentiation of sildenafil and increased adverse reactions. There is an urgent need for pharmocokinetic studies of the concomitant administration of sildenafil and highly active antiretroviral therapy (HAART).

References:

1) The Pharmaceutical Journal Vol. 262, p. 423, 27 March 1999.
2) Daselli D, Comolli G, Maccabruni A et al. Possible interaction between sildenafil and HIV combination therapy. Lancet 353:840 (1999).