A weight loss pill sold over the internet and in Chinese medicine shops has alerted the Medicine and Healthcare products Regulatory Agency (MHRA) to issue a warning to consumers.
MHRA says anyone taking Herbal Flos Lonicerae (Herbal Xenicol) should stop immediately and contact a doctor, reports the BBC.
The warning comes after reports of side effects such as palpitations, with one patient needing hospital treatment.
The capsules are thought to contain an as yet unknown pharmaceutical substance. Tests are underway to establish the cause of the side effects.
By April this year, under a European directive, all manufactured herbal medicines will have to be registered under a new scheme to prove they meet safety and quality standards.
The scheme is known as the Traditional Herbal Registration (THR).
"Since 2005, the MHRA has found over 280 examples where so-called 'herbal' or 'natural' products have been adulterated with random quantities of powerful pharmaceutical substances. Such products pose a real risk to the public," Richard Woodfield, head of herbal medicines policy at the MHRA, said.