Melanotan is a synthetic hormone which stimulates the body's production of melanin, a substance that gives the skin its colour. And it is being offered as Melanotan I and Melanotan II.
Experts are worried that the long-term effects are not yet known.
A BBC investigation found needle exchanges across the UK have noted more and more people coming in to get syringes to inject the tanning drug.
Melanotan has been developed for the treatment of skin conditions but has not yet been licensed for human use.
It is sold illegally online and in salons and gyms It is not against the law to use the drug, and Stacey Boardman, from Rochdale, said she was willing to take the risk.
You have about 10 injections. You take an injection every day and after the 10 days you should have a nice tan," she said.
"And then if you want to go darker just carry on taking it for more than 10 days. Once you've got your initial tan, then just once a week to keep that colour you're happy with."
The government's Medicines and Healthcare products Regulatory Agency (MHRA) has written to 18 websites in the past few months warning them to stop advertising the product.
Claire Tilstone, from the MHRA, said the potential dangers were two fold.
"It has not been tested for its effectiveness or its safety," she said.
"We do not know what side effects it may cause and potentially what the long-term consequences on their health may be."
She added there were also concerns about users injecting themselves without proper medical advice.
David Carter, head of the MHRA's medicines borderline section, said, 'We are warning people not to use this product.
'Don't be fooled into thinking that Melanotan offers a shortcut to a safer and more even tan. The safety of these products is unknown and they are unlicensed in the UK. The side effects could be extremely serious.
'If you have used either of these products do not use them again and if you have any concerns you should seek advice from your doctor.'
Melanotan has to be diluted with bacteriostatic water (a prescription-only medicine) before it can then be self-injected into the skin, according to the MHRA.
Often the product is in its concentrate state. However, the MHRA has seen some websites offering to reconstitute the product prior to sending it to the consumer.
In the UK, there are licensed water products for the injection of medicine, including bacteriostatic water.