 7. Sildenafil citrate Viagra. Sildenafil citrate doses 25mg should not be taken within 4 hours of taking an alpha blocker or it can produce significant and symptomatic hypotension Pfizer. Of note, Viagra, like buprenorphine, is metabolized by cytochrome P450 3A4. Ingestion of buprenorphine could potentate the effects of sildenafil citrate which could extent the period of safety to 4 hrs from when an alpha blocker could be taken after Viagra ingestion. 