Members of the public have been warned to avoid a nasal spray that claims to help people get a tan.
Ubertan, which is being sold illegally over the internet and in gyms and beauty parlours in the UK, is said to contain a substance that has side-effects which could put people's health at risk.
The product has been described by the Medicines and Healthcare products Regulatory Agency (MHRA) as "potentially dangerous" after tests on the product showed it contained an unlicensed compound called melanotan II.
The substance is supposed to increase how much melanin is produced in the body, making your skin go darker, but the MHRA said that there is no evidence to show it works or that it is safe.
The MHRA also tested another version of the spray, which was found to contain the unlicensed herbal extract forskolin, which can potentially lower blood pressure and promote secretion of hormones from the thyroid.
Head of the MHRA's medicines borderline section David Carter said: "None of these products have been clinically tested and as a result there is no way of knowing just how serious the side-effects could be, or the impact on health."