A drug which dealers claim promises a "herbal high" was declared dangerous and illegal by the UK's medicines regulator today.
Benzylpiperazine (BZP) - also known as Pep Love, Cosmic Kelly and The Good Stuff - comes in pill form, often imprinted with a housefly, heart or butterfly.
Although some producers claim it comes from natural pepper extract, it is synthetic and can cause vomiting, abdominal pain, seizures and abnormal heart rhythms.
BZP contains piperazine which is used as an anti-worming agent and is found in human and veterinary products.
The warning came from the Medicines and Healthcare products Regulatory Agency (MHRA), which said vendors of the drug may face prosecution.
Spokesman Danny Lee-Frost said: "Producers and sellers must now stop marketing all products containing benzylpiperazine and piperazine blend in the UK with immediate effect and must cancel all advertising and promotion including internet promotion and sales.
"People should not take these pills as there are considerable health risks."
The MHRA said in rare cases BZP - which has side-effects similar to amphetamines - could cause death.
Mr Lee-Frost said piperazine-based products were classified as medicinal products which must only be sold under licence in a pharmacy.
But BZP pills are unlicensed and there is there no safeguard over the quality or safety of the drug, he added.
BZP also has been found for sale on the internet in powder or liquid form.
If someone is convicted under the Medicines Act 1968, they can be sentenced to a maximum of two years in jail.