People are being warned against buying "dangerous" herbal slimming aids online.
The Medicines and Healthcare products Regulatory Agency (MHRA) said there were concerns about some products, including Instant Slim and Sport Burner, being sold on websites and shipped around the world.
Some contain the prescription-only drug sibutramine, which has been withdrawn across Europe due to an increased risk of heart attacks and strokes.
Safety warnings about herbal slimming aids have been issued by several international agencies.
The Swedish Medical Products Agency has issued warnings over Slimline Soft Gel, Acai Berry ABC, Hygia Fit, Sport Burner, Health Slimming Coffee, Instant Slim, 3X Slimming Powder, Li D Daidaihua and Botanical Slimming.
In the US, experts have issued a warning over Celerite Slimming Tea, while in Australia, Pink Lady for Women capsules and St Nirvana Herbal Slimming capsules are under scrutiny.
The Hong Kong Department of Health has issued warnings over more than a dozen products, including a range called Super Fat Burning Bomb.
MHRA group manager for herbal medicines policy, Richard Woodfield, said: "People often assume that because herbal medicines and products contain natural extracts they are automatically safe but this is not the case - no medicine or product is risk free.
"We urge people not to take any slimming medicines or products bought online without consulting with a pharmacist or doctor as they risk being supplied with medicines that are not safe or not suitable for them to use.
"People can also protect themselves by making sure they look out for the Traditional Herbal Registration (THR) logo.
"Products and medicines bearing this logo have been assessed and quality checked to ensure that they are safe to use.
"It simply is not worth the danger to overall health to buy and use these products as you just don't know what is in them and any weight loss results they offer could come with such huge risk."