It's a little pill that promises hope to the pale, the flabby and the limp.

No wonder the Melbourne company developing what has been been nicknamed the "Barbie drug" believes it will have sales of $1bn (£533.5m) a year when it goes on the market in 2006.

Called Melanotan, the drug was developed and will be promoted as a way of preventing skin cancer.

But its side-effects include an artificial tan, increased libido and weight loss.

Wayne Millen, the chairman of Epitan, which is developing the drug, believes it could achieve huge sales, despite being available on prescription only.

"I'm sure it will be just like how people get steroids, botox and Viagra." he said.

"You go along and say you want protection against skin cancer and say what you like to get the prescription. We don't mind, because ethically if they produce melanin they're protected. They can tell whatever story they like to their physician."

Melanotan uses an artificial protein to stimulate melanocytes, the skin cells which produce melanin. Melanin is responsible for skin pigmentation and screens out some of the ultraviolet rays which cause skin cancer.

Mr Millen said the results of a trial conducted in Australia last year were expected to be published in an issue of an American dermatology journal and had been presented recently at a scientific conference in Germany.

He said the drug should be seen only as a back-up.

"We are advising people against staying out longer in the sun, but while you are out there this will help your protection," he said.

But doctors are worried. Craig Sinclair, of the Cancer Council of Australia, said Melanotan could reduce people's wariness about exposing themselves to the sun.

"If the drug was effective and had no side-effects it would be like a [factor] 4-8 sunscreen. It certainly won't in any way replace the need for effective protection."

Terry Piva, an authority on skin cancer at the Royal Melbourne Institute of Technology, said people would spend more time in the sun if they thought the risk of cancer was reduced.

"It's a false sense of security," he said.

He said that melanocyte-stimulating drugs should not be used until more research had been done.

"I'd be concerned about the effects of long-term use, because you're interfering with a natural hormone in the skin. We don't know what effect that has."

Australia has one of the highest rates of skin cancer in the world, half the population developing it at some point in their lives.

About 1,000 people die from skin cancer each year.

Robin Marks, professor of dermatology at Sydney University, said that promoting the health effects of a suntan would go against 20 years of cancer prevention policy.

"The biggest problem people had in reducing the community's desire for a suntan was to remove the link between a tan and health," he said.

He said too little information about the drug had been published in medical journals.

"I'm used to determining the value of a product on the basis of well-designed scientific studies which have been performed and the results submitted for peer review.

"I have seen nothing like that for this product."