To determine the efficacy of sildenafil citrate (Viagra) in patients with erectile dysfunction after three-dimensional conformal external beam radiotherapy for prostate cancer.

Sixty patients treated with radiotherapy for prostate cancer at least 6 months previously, complaining of erectile dysfunction, and not using nitrates were entered into a double-blind, placebo-controlled, crossover study lasting 12 weeks. They received 50 mg of sildenafil or placebo for 2 weeks; at week 2, the dose was increased to 100 mg in the case of an unsatisfactory erectile response. At week 6, patients crossed over to the alternative treatment. Patients were then allowed to enter a 6-week open-label phase using 50 mg of sildenafil in the first 2 weeks, increasing the dose to 100 mg in the case of an insufficient erectile response in the following 4 weeks. Data were collected before trial entry (baseline evaluation) and every 2 or 4 weeks using the International Index of Erectile Function questionnaire. Any side effects were recorded. Two years later, all patients were approached by mail to evaluate their current sexual functioning and possible use of sildenafil.

All patients completed the double-blind, crossover study. For nearly all the International Index of Erectile Function questions, sildenafil caused a significant increase in mean scores from baseline. Placebo had no effect. Seventy-seven percent entered the open-label phase; they were the better responders in the double-blind phase. Sildenafil was as equally effective as in the double-blind phase. The side effects were mild or moderate and significantly decreased in the open-label period. Two years after trial entry, 24% still used sildenafil.

Sildenafil is relatively effective in patients with erectile dysfunction after three-dimensional conformal external beam radiotherapy for prostate cancer. New drugs and combination therapies are needed to improve outcome.