Retailers of the tanning nasal spray Ubertan are under investigation by a government watchdog following an outcry over the product's potentially serious side effects.

The owners of ubertan.com – along with two other sellers – could now face a maximum two-year prison sentence following legal action by the Medicines and Healthcare products Regulatory Agency (MHRA).

Ubertan, which was being sold illegally in gyms and tanning salons, is no longer available after the MHRA recently forced the company to close its UK-based website.

Ubertan contains melanotan, a chemical that stimulates the production of melanin in skin cells making it darker. Reported side effects include nausea, migraines, dizziness, palpitations and enhanced libido.

The manufacture of products containing melanotan is not illegal but their sale and advertising is. Nonetheless, such products are freely available on the internet. An MHRA spokesman confirmed that it is now seeking to target "all melanotan products".

He added: "The MHRA continues to monitor retailers advertising and selling this unlicensed medicine and will take the appropriate action in order to safeguard public health.

"We have investigated nine different retailers of Ubertan, six of which have complied following urgent notices dispatched to them. We have three cases currently ongoing.

"We strongly advise people not to use this product. Ubertan or any other melanotan-containing product are unlicensed medicines, and as such the safety, quality and effectiveness has not been demonstrated and could have serious side effects."

Al Zabiela, 56, owner of Al'z Gym, Oldham, and wife Amanda, 42, have both used nasal sprays containing melanotan. When an Ubertan brochure arrived at Zabiela's premises six months ago, he asked his wife if she would like to try the product.

Zabiela said: "The leaflet was really professional. It didn't say too much, but it did say that the product was a collection of amino acids. It all seemed above board. But it didn't really work at all, so that was it. I daresay if it would have worked we would have been selling it now."

Amanda said: "I have used the [melanotan nasal sprays] a couple of times in the past six months. They stop you from burning when you go on holiday, and deepen your natural tan.

"It is down to the individual if they want to try it – it doesn't concern me."

Zabiela said the "vast majority" of his clients who use the sprays experience nausea and huge fluctuations in their sex drive.

"I have got some guys who use them without going in the sun – it makes you a really grey shade of brown and gives you rings under the eyes. But once you go in the sun, it's fantastic."

Candy Garvey, 38, manages the Tanning Room in Lees, Oldham. An Ubertan representative had tried to sell her the product.

"We said we weren't interested," she said. "My personal opinion is that it is not safe. If it is not tanning you on the outside, what is it doing to your insides?

"People go too far to get a tan. They need to think about the health implications of what they are doing and the long-term damage."

Dispensing assistant Mary Roach, 58, of Watersheddings, Oldham, said she uses sunbeds for health reasons.

"When I go on a sunbed, my skin produces melanin. And if I go abroad, as I am in the next few weeks, it stops me from burning. I only go under sunbeds for six minutes, and I always wear suncream.

"These products are totally misleading. They are used for totally cosmetic purposes."

The British Association of Beauty Therapy & Cosmetology, an industry body, said Ubertan is not a product it covers in its insurance policy nor endorses in any way.

Dr Claire Knight, health information officer at Cancer Research UK, said: "Using melanotan in the quest for a tan is risking your health. If your heart's set on getting a tan, then using fake tan is a safer way to do it."