Thousands of people are understood to be ordering the drug Melanotan, which is used to darken the skin, over the internet. The Therapeutic Goods Administration has not registered Melanotan for use in Australia and it is classified as a prescription-only medicine. Doctors warn it can accelerate the development of skin cancers.

Importing Melanotan without a doctor's approval can lead to a $170,000 fine and 12 months' jail.

Customs and Border Protection last year seized 48 shipments of Melanotan-type products the TGA declared illegal.

But sources who use the product regularly have told Fairfax Media that underground use of the drugs is rife. One user, Sarah, agreed to talk on condition of anonymity. The 19-year-old said she gets the injections at a friend's house every week, and they cost between $1 to $10 a shot.

Sarah, who works as a promotions model, said she preferred injections to sunbeds or lying on the beach.

''Everything goes darker, including your freckles, but there are no tan lines and you don't get burnt,'' she said. ''Pretty much everyone I know uses it … even some of the guys.''

The tanning drug has the side-effect of causing prolonged erections in some men.

The chief executive of Cancer Council Australia, Ian Olver, said using the drug could increase the risk of skin cancer on existing moles as they became more pronounced, and injecting any illegal substance increased the danger of blood-borne viruses. ''It's a dangerous thing to do - to take a drug that hasn't been fully tested and inject it into yourself. It's not known if it's safe because it has not been rigorously tested in the way a commercial drug would be,'' Professor Olver said.

But body-building forums are promoting the injections as a safe alternative to sunbeds. Online, 10-milligram bottles of Melanotan II sell for $69 plus postage.