Pharmaceutical company Pfizer promises a weight-loss pill that comes without side effects and could be on the market within three years.

Pfizer has begun clinical trials on a compound which mimics the effect of a hormone that switches off appetite and the enjoyment of eating.

The compound – codename OAP-189 – is a synthetic version of a hormone called oxyntomodulin, the effect of which was discovered by researchers at Imperial College London.

Drugs already on the market which promote weight loss, including Xenical or Alli, have unpleasant side effects such as diarrhoea, wind and even incontinence.

OAP-189 could bring Pfizer the financial success it enjoyed with Viagra, which it launched in 1998.

The time between successful trials and drugs being available in shops can be last little as three years.

Scientists at Imperial College found that patients who had stomach bypass operations producer higher levels of appetite suppressing hormones, including oxyntomodulin, The Sunday Times reported.

Appetite suppressing hormones convince the brain the body does not need food.

Stephen Bloom, professor of investigative medicine at Imperial College, told the newspaper: “I think we could mimic the dramatic weight loss achieved with stomach bypass surgery by giving people gut-hormone derived therapies.

“If you take away hunger, food is not attractive. If you take away pleasure, people stop eating.”

More than 130 drugs have been tested as anti-obesity agents, but most have been ineffective long-term or had unacceptable side effects.

Other products, Rimonabant and Reductil, have been withdrawn over safety fears after going on sale.

Tam Fry, a spokesman for the National Obesity Forum, told The Sunday Times: “Using these hormones to try to treat the overweight has to be the way forward because they are having an effect on the brain.

“We think this approach offers the prospect of the breakthrough we need. We just have to hope it doesn’t take too long to get one of them on the market.”

The research at Imperial College also found three other hormones produced in the digestive system could also be effective in weight loss.