Sildenafil (Sildenafil citrate, Viagra^®)

A new drug developed to target sexual impotence has sent men flocking to pharmacies and doctors' surgeries all over the United States.

Viagra has been prescribed to 40,000 men in the first three weeks it has been on sale in the US.

British men are also eager to get their hands on the pill, with the British Impotence Association reporting an average 100 calls and 100 letters each day enquiring about Viagra. The drug is expected to go on sale in Europe later this year.

Doctors in the UK have warned that the pills are not suitable for everyone and should only be taken if prescribed.

Up to 40% of men are believed to suffer from impotence by the age of 40.

The drug was originally developed to improve blood supply to the heart in angina sufferers. It has now been adapted to facilitiate the flow of blood to the penis necessary to sustain an erection.

By inhibiting the phosphodiesterase 5 (PDE5) enzyme, which is found mainly in the penis, Viagra allows the chemical produced during sexual stimulation, cyclic GMP, to persist.

The longer cyclic GMP persists, the better the chance for increased blood flow.

5-[2-Ethoxy-5- (4-methylpiperazin-1-ylsulfonyl) phenyl]-1-methyl-3-propyl- 6,7- dihydro -1H- pyrazolo[4,3-d]pyrimidin-7-one

Formula : C22 H30 N6 O4 S

The light-blue, diamond-shaped Viagra tablets are being offered for sale over the Internet at a cost of $105 for 10.

Pharmacodynamics

Male Reproduction

Clinical studies: sildenafil enhanced erectile response in patients with erectile dysfunction associated with spinal cord injury, according to the results of a UK-based trial. 27 men with spinal cord injury and erectile dysfunction were randomised to receive sildenafil 50mg, or placebo, approximately 1h prior to intercourse. After a 28-day study period, 9/12 sildenafil recipients, compared with 1/14 placebo recipients, reported an improvement in their erections. In addition, sildenafil, compared with placebo, recipients reported significantly greater satisfaction with their sex life. Eight of the 12 sildenafil recipients, compared with 2/13 placebo recipients, indicated that they would be interested in continuing therapy.

Adverse Events

In an open-label trial in 311 men with erectile dysfunction, adverse events included headache (11% of patients), facial flushing (4%) and indigestion (<4%). There were no signs of tachyphylaxis, and only 1 case of visual disturbance. Patients in previous dose-escalation trials reported seeing a blue haze and transient increased brightness when taking sildenafil doses over 200mg