Viagra, Cialis and Levitra are the three oral drugs approved for the treatment of erectile dysfunction or impotence here.

Known as PDE-5 (phosphodiesterase type-5) inhibitors, they suppress the PDE-5 enzyme in the body. This increases blood flow to a man's penis so that he will get an erection when sexually stimulated.

They are usually taken 30 minutes before sex. All three are available only by prescription, and cost about $20 a pill.

According to health information website WebMD, Levitra works a little longer than Viagra. They both take effect in about 30 minutes. With Levitra, the effects last for about five hours. With Viagra, the effects last approximately four hours.

Cialis works a bit faster (within about 15 minutes), and the effects last much longer - up to 36 hours in some cases.

THE FAKE

There are hundreds of them out on the market, but the ones which made the news recently are Power 1 Walnut, Fake Cialis, Santi Bovine Penis Erecting Capsule and Zhong Hua Niu Bian.

Available in some shops and the back lanes of red-light districts, they are illegally manufactured in countries such as China, Indonesia and India.

Many contain potent substances. Power 1 Walnut for instance has traces of sildenafil - the key ingredient in Viagra - but dangerous levels of glibenclamide, a prescription drug for diabetes which can result in lethally low blood sugar levels.

There have been more than 30 confirmed, and 70 suspected, cases of adverse reactions from these fake drugs. Two victims fell into a coma and died, while scores of others were hospitalised for nausea, vomiting and dizziness.

The drugs come in all colours, shapes and sizes although the fake Viagra, Cialis and Levitra pills are made to look like the real thing.

These potentially lethal drugs cost between $1 and $5 a pill.

This article was first published in The Straits Times on May 3, 2008.