An unlicensed "tanning injection" could leave users suffering serious side-effects, a British regulator has warned.
Melanotan was developed to help treat skin disorders and has not been licensed for human use.
But it is believed the drug – which users inject – is being sold in increasing quantities on the internet and in some tanning salons and body building gyms.
The synthetic hormone stimulates the body's production of melanin, the substance which gives skin its color.
David Carter, head of the Medicines Borderline Section at the Government's Medicines and Healthcare Products Regulatory Agency in the U.K., said the long term side-effects are not known.
"Don't be fooled into thinking that Melanotan offers a shortcut to a safer and more even tan," he said.
"The safety of these products is unknown and they are unlicensed in the U.K. The side effects could be extremely serious.”