An investigation by consumer champion Which? has found that melatonin supplements are being illegally advertised by multiple retailers including Amazon, eBay, Superdrug Marketplace, and Temu.

Melatonin, a hormone that plays a role in regulating sleep, is only available with a prescription in the UK.

Which? said it is illegal to market melatonin without a prescription in the UK as it can interfere with common medications and cause unpleasant side effects.

According to the NHS, the usual starting dose of melatonin doctors would prescribe is 2mg. But Which? consistently found 10mg listed for sale, which is the maximum dose usually recommended by the NHS for people suffering with long-term insomnia.

During the investigation between October 2025 and January 2026, Which? even saw one supplement for sale on Temu which claimed to contain 30mg of melatonin, around 15 times the usual starting dose in the UK.

In many of the listings Which? found, melatonin was not mentioned in the image or product name but was only visible on the list of ingredients.

Which? said it found eight supplements claiming to be “night time fat burners” on eBay that listed melatonin among their ingredients, with one product not disclosing the amount of melatonin it contained.

Sellers also hid that products contained melatonin in their listings to stop the platforms detecting and removing the products.

The investigation found sellers on Temu used spaces to get around automatic detection by naming the product “melat onin” or only listing it as an ingredient.

Eight “melat onin” products were advertised on Temu in November, but all had been removed by January.

Sellers are finding new ways to avoid detection, with the consumer champion finding several supplements marketed on eBay and Temu that hid melatonin from the product listing, instead only mentioning it in the less prominent product description.

Which? also saw listings for melatonin skin patches and teas, which seemed to avoid detection by online platforms and health retailers.

These were sold through third party sellers on Amazon and Superdrug Marketplace.

Which? warned that if shoppers choose to buy melatonin from illegal sellers, there is no knowing what they will end up with as the sellers are operating outside legal channels.

Responding to the investigation, a spokesperson for Temu said that it prohibits the sale of products containing melatonin and removes any such listings once detected, with penalties applied to sellers for violations.

“Our monitoring systems continuously screen the platform for non-compliant products," they continued. "As you noted, many of the melatonin products you identified had already been detected and removed before your inquiry. The remaining products were removed on the same day you flagged them, and the sellers penalised."

Superdrug said that topically applied products that contain melatonin as one of their ingredients are "widely available" for general sale across the UK.

“We have contacted the Medicines and Healthcare products Regulatory Agency (MHRA) for clarification on the guidelines for including melatonin as an ingredient within topically-applied products," it added.

A spokesperson for eBay said that consumer safety is a top priority for the company and that it has removed the items.

“We work diligently to prevent and remove unsafe product listings through seller compliance audits, block filter algorithms, AI-supported monitoring by in-house specialists, and close partnerships with regulators," added the online marketplace. "Together, these measures help to prevent millions of potentially unsafe items from being listed each year.

“We are reviewing the wider marketplace to remove any identical listings. We are continually updating filters to prevent bad actors from circumventing our policies."

Responding to the findings, Amazon said that safety is its top priority, and it requires all products offered in the store to comply with applicable laws, regulations and Amazon policies.

“We develop innovative tools to prevent unsafe products from being listed and if we discover a product was undetected by our automated checks, we address the issue immediately and refine our controls,” an Amazon spokesperson said. “We take action to maintain a safe selection for our customers, including removing noncompliant products, and outreach to sellers, manufacturers, and government agencies for additional information, when appropriate.”

Which? called for better oversight of the supplement industry to ensure illegal or unsafe products are taken off the market.

It urged the government to put strong enforcement in place so that any companies breaking the rules know they will be held to account.

“If shoppers buy from sellers illegally advertising melatonin, there is no telling what the products will contain - they could contain too much melatonin or other potentially harmful substances, which could lead to dangerous side effects,” Sue Davies, Which? head of consumer policy, said. “Better oversight of the industry is desperately needed so the government and regulators can crack down on illegal listings and ensure that any sellers who break the rules are held to account.”

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