At least 100 million tablets of sildenafil have been prescribed worldwide (Pfizer's data; personal communication). The use of sildenafil has only rarely been associated with priapism, 1 2 although one such case was also associated with sickle cell trait.2 We report two cases of interaction between dihydrocodeine and sildenafil that resulted in prolonged erections.

Case 1—A 49 year old married man presented to our sexual function clinic with erectile dysfunction after a fall in which he sustained thoracic vertebral fractures. His partial paraplegia improved over the course of a year, after which he presented for treatment of erectile dysfunction. He was successfully treated with 100 mg of sildenafil, which produced hard erections that detumesced immediately after intercourse. He later sustained a minor soft tissue injury to his right shoulder, for which he was initially given non-steroidal anti-inflammatory drugs. He continued to experience pain during the treatment, so his family practitioner changed the treatment to dihydrocodeine 30 mg six hourly, which largely controlled the pain. While taking dihydrocodeine he also took 100 mg of sildenafil. Although he had an orgasm and ejaculated, his erection remained hard for five hours and then subsided. Four days later he again took sildenafil while taking dihydrocodeine, after which he had an erection that lasted four hours. He has since stopped taking dihydrocodeine. He continues to use sildenafil 100 mg, which produces erections that subside immediately after orgasm.

Case 2—A 37 year old homosexual man with HIV was being followed up at our unit. His HIV viral load was 4125 RNA copies/ml and his CD4 cell count was 860 cells/mm³ (23%). He was not receiving antiretroviral treatment. The patient was referred to the sexual function clinic, where he was diagnosed as having psychogenic erectile dysfunction. He had recently been treated for moderate depression with cognitive behaviour therapy. While taking 100 mg of sildenafil he had hard erections that subsided after orgasm. He later took dihydrocodeine (30-60 mg six hourly) for a soft tissue injury. During the first seven days of the analgesic treatment he took 100 mg of sildenafil three times, which resulted in a prolonged erection lasting two to three hours after orgasm on each occasion. He continued to take the dihydrocodeine regularly for two more weeks, and the erection he had while taking sildenafil during this time detumesced immediately after orgasm.

The effects of opiates on sexual functioning are inconsistent and variable, but lowered testosterone concentrations, reduced sexual desire, erectile dysfunction, and delayed ejaculation have been frequently reported, particularly among chronic narcotic drug misusers. 3 4

The acute opiate intake in our patients on sildenafil produced abnormally high cyclic guanosine monophosphate concentrations in peripheral nerve endings, which resulted in prolonged erections even after orgasm.

The manufacturers have no reports to date of prolonged erections in patients taking acute dose opiates and sildenafil, apart from our two cases. It is important when prescribing sildenafil to ask about current use of opiates, and to warn about such concurrent use in the future.