The jab boosts levels of the body's natural pigment
An unlicensed medicine which can be injected to produce a tan may be responsible for worrying changes in skin moles, say doctors.
Two women told Salford dermatologists that they had both injected "Melanotan" shortly before noticing the problem.
Medicines watchdogs have issued warnings about the untested nature of the internet-supplied jabs, the British Medical Journal reports.
The British Skin Foundation said there were better options for a safe tan.
Melatonan jabs work by boosting the levels of melanin, the body's natural pigment which produces a tan.
A growing number of people in the UK are injecting themselves with the jabs, a BBC investigation found last November.
Both the patients who visited the Salford Royal Hospital were regular sunbed users with "conspicuous" tans.
One, a 42-year-old woman, had two moles on her foot which had increased in size and darkened over just a few weeks.
The other, who was 30, told doctors that the moles on her back had also rapidly darkened.
Both then told doctors they had injected the tanning jab in recent weeks.
Although rapidly changing moles can be a sign of skin cancer, none of the moles were found to be malignant.
However, the doctors said that more and more women were likely to seek medical help for the problem, and asking about the tanning jab could reveal the reason for the changes.
The Medicines and Healthcare Regulatory Authority recently broadcast their concerns about the drugs, which are not licensed for use in the UK.
They said that it was possible the injections could be contaminated, carry infection, or be responsible for passing it on, where women chose to share needles.
Doctors warn against sunbathing, which is linked to skin cancer
A spokesman for the British Skin Foundation said that the latest report should "serve as a strong warning" that people should avoid the jabs.
"People may think that this drug is a safe alternative to sunbeds and sunbathing because it doesn't require UV light.
"They may even think it offers sun protection, but the protection offered by a natural tan is minimal, and when it comes to an injectable, unlicensed drug, we have no idea what side effects it may incur.
"The product hasn't yet undergone the necessary safety tests to be legally sold in this country.
"There are other ways to get a tan without the risks of using an unregulated product, so until it gets the all clear for use on humans, stick to spray tans and self tanning lotions instead."