By Tom Mangold BBC Two's Correspondent

It has no known side-effects, and contains a molecule that fools your brain into believing you are full.

Deep inside the African Kalahari desert, grows an ugly cactus called the Hoodia. It thrives in extremely high temperatures, and takes years to mature.

The San Bushmen of the Kalahari, one of the world's oldest and most primitive tribes, had been eating the Hoodia for thousands of years, to stave off hunger during long hunting trips.

When South African scientists were routinely testing it, they discovered the plant contained a previously unknown molecule, which has since been christened P 57.

The license was sold to a Cambridgeshire bio-pharmaceutical company, Phytopharm, who in turn sold the development and marketing rights to the giant Pfizer Corporation.

Fortune cactus

A molecule in the cactus makes you feel full

When I travelled to the Kalahari, I met families of the San bushmen.

It is a sad, impoverished and displaced tribe, still unaware they are sitting on top of a goldmine.

But if the Hoodia works, the 100,000 San strung along the edge of the Kalahari will become overnight millionaires on royalties negotiated by their South African lawyer Roger Chennells.

And they will need all the help they can to secure the money.

Currently, many bushmen smoke large quantities of marijuana, suffer from alcoholism, and have neither possessions nor any sense of the value of money.

The truth is no-one has fully grasped what the magic molecule means for their counterparts in the developed world.

Blood sugar

According to the British Heart Foundation 17% of men and 21% of women are obese, while 46% of men and 32% of women are overweight.

So the drug's marketing potential speaks for itself.

Phytopharm's Dr Richard Dixey explained how P.57 actually works:

"There is a part of your brain, the hypothalamus. Within that mid-brain there are nerve cells that sense glucose sugar.

"When you eat, blood sugar goes up because of the food, these cells start firing and now you are full.

"What the Hoodia seems to contain is a molecule that is about 10,000 times as active as glucose.

"It goes to the mid-brain and actually makes those nerve cells fire as if you were full. But you have not eaten. Nor do you want to."

Clinical trials

Dixey organised the first animal trials for Hoodia. Rats, a species that will eat literally anything, stopped eating completely.

When the first human clinical trial was conducted, a morbidly obese group of people were placed in a "phase 1 unit", a place as close to prison as it gets.

All the volunteers could do all day was read papers, watch television, and eat.

Half were given Hoodia, half placebo. Fifteen days later, the Hoodia group had reduced their calorie intake by 1000 a day.

It was a stunning success.

The cactus test

In order to see for ourselves, we drove into the desert, four hours north of Capetown in search of the cactus.

Once there, we found an unattractive plant which sprouts about 10 tentacles, and is the size of a long cucumber.

Each tentacle is covered in spikes which need to be carefully peeled.

The San will finally throw off thousands of years of oppression, poverty, social isolation and discrimination Roger Chennells, lawyer

Inside is a slightly unpleasant-tasting, fleshy plant.

At about 1800hrs I ate about half a banana size - and later so did my cameraman.

Soon after, we began the four hour drive back to Capetown.

The plant is said to have a feel-good almost aphrodisiac quality, and I have to say, we felt good.

But more significantly, we did not even think about food. Our brains really were telling us we were full. It was a magnificent deception.

Dinner time came and went. We reached our hotel at about midnight and went to bed without food. And the next day, neither of us wanted nor ate breakfast.

I ate lunch but without appetite and very little pleasure. Partial then full appetite returned slowly after 24 hours.

The future

Mr Chennells is ecstatic:

"The San will finally throw off thousands of years of oppression, poverty, social isolation and discrimination.

"We will create trust funds with their Hoodia royalties and the children will join South Africa's middle classes in our lifetime.

"I envisage Hoodia cafes in London and New York, salads will be served and the Hoodia cut like cucumber on to the salad.

"It will need flavouring to counter its unpleasant taste, but if it has no side effects and no cumulative side-effects."

Unfortunately for the overweight, Hoodia will not be around for several years, the clinical trials still have several years to run.

Do not travel to the Kalahari to steal the cactus as it is hard to find and illegal to export.

And beware internet sites offering Hoodia "pills" from the US as we tested the leading brand and discovered it has no discernible Hoodia in it.

So just be patient. Help is at hand.

BBC Two's Correspondent was broadcast on Sunday, 1 June, 2003 at 1915 BST.