Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor and prevents the breakdown of cyclic guanosine monophosphate (cGMP). During sexual stimulation, cGMP is produced in the penis, relaxing muscle in the corpora cavernosa so that blood can flow into the corpora and produce an erection. Sexual stimulation is still needed to produce an erection.

No more than one dose of OTC sildenafil is to be taken per day. One 50mg tablet should be taken with water approximately one hour before sexual activity. It normally takes between 30 to 60 minutes to work; if required, it can be taken up to four hours before sexual activity. Sildenafil can be taken with or without food but fatty meals may delay absorption and thus delay onset of effect. Grapefruit and grapefruit juice inhibit CYP3A4 enzyme and can lead to higher sildenafil levels in the blood. 

Most men find that sildenafil works after one or two doses (taken on separate days). Some may need to take sildenafil a number of times on different days (a maximum of one 50mg tablet per day) before they can achieve an erection satisfactory for sexual activity. If a patient is still not able to achieve an erection sufficient for satisfactory sexual activity, they should be advised to consult with a doctor. 

Most side effects are mild or moderate and of short duration. The commonest are headache, which may affect more than one in 10 men, and indigestion/dyspepsia. Nausea, stuffy nose, dizziness, facial flushing, hot flush, colour tinge to vision, blurred vision, and visual disturbance are also seen. If visual impairment occurs the drug should not be used again.