By Jane Dreaper
BBC News, health reporter
The UK medicines regulator is warning people to stay away from pills called BZP, the BBC has learned.
Benzyl Piperazine is advertised as a 'legal high'
The pills are said to offer a "legal high", but the Medicines and Healthcare products Regulatory Agency (MHRA) has warned they can cause health problems.
It has seized thousands of pills in the past year and plans to prosecute importers, sellers and manufacturers.
Many shops have withdrawn the pills from sale, but some claim BZP helps keep people away from hard drugs.
BZP stands for Benzyl Piperazine - the main active ingredient is piperazine, which is more commonly used as an anti-worming medicine, in animals and humans.
But it also acts as a stimulant - giving similar effects to amphetamines, or speed.
That means health risks too - such as vomiting, dizziness and allergic reactions.
Danny Lee-Frost, from the MHRA, investigates the sale of unlicensed medicines.
He said: "Our main concern is that people are being led to believe these are safe.
"They walk into a shop and see a sign that says 'legal alternative'.
"But it's not legal and it's not an alternative - it's a dangerous medicinal product in its own right. People should not take it."
Sue Harrison agrees. At her shop in a busy market in Manchester, she rues the day when a business contact gave her what she thought was an energy tablet.
"Within five minutes I began to feel extremely ill," she said.
"I was just getting worse and worse and they decided to put me on a hospital ward - I really thought at that time that I was going to die.
"I know it sounds very dramatic but something like this had never happened to me before in my life.
"It takes me all my time to have a paracetamol tablet - never mind something that would harm me."
Sue Harrison spent two days in hospital and it was another fortnight before she returned to work.
She is 44 and had never wanted to experiment with drugs.
"I'm just very disappointed in myself - it's something I will never ever do again.
"I relive it psychologically through flashbacks - but I need to highlight this to other people never to do this."
Regulators want to make sure that clubbers and people going to the summer music festivals are aware of the risks of any substance they may be offered.
Some shops that had been selling BZP have now voluntarily withdrawn the pills, after being told about the medicine agency's investigations.
But there are claims that BZP had a useful function in keeping people away from hard drugs.
One shopkeeper, who didn't want to be identified, had sold BZP and tried it himself, with no bad effects.
"It is a big effect - it's a lot of energy. You feel it actually coming through your body, like a hot rush.
"On comparison to illegal Ecstasy it's the same price. It's not as strong as the street drugs, but it's a way of getting people off them."