The fashionable party drug mephedrone has been linked to up to 98 recent deaths in Britain, government advisers have warned in calling for tougher action to combat the proliferation of legal highs.
The Advisory Council on the Misuse of Drugs said unscrupulous manufacturers made a mockery of the law by falsely advertising addictive substances as "plant food" or "bath salts".
Its chairman, Professor Les Iverson, said young users of "designer drugs" were playing "Russian roulette" with their lives - and the effects were already being seen in hospitals.
"We are not seeing just a nice party drug but something that can kill."
Iverson released figures showing that in the past two years mephedrone had been confirmed as a factor in 42 deaths and had not been ruled out as contributing to a further 56.
Users of designer drugs - created in labs to mimic the make-up of banned substances such as Ecstasy and amphetamines - suffered such extreme side-effects that they had to be sedated.
They had also been treated for paranoia, psychosis, high heart rates and raised blood pressure, he said.
"Users are playing Russian roulette. They are buying substances marked as research chemicals. The implication is that you should do the research on yourself to find out whether they're safe or not. This is a totally uncontrolled, unregulated market."
The first large quantities of legal highs, or psychoactive drugs - many made in China - appeared in Britain two years ago.
They can be easily bought online or from shops selling drug paraphernalia and herbal goods. Some undergraduates also sell them to fellow students.
The council said: "Many people importing these new substances appear to have had no previous involvement in the illicit drug trade and are just in it to make a quick buck."
Ministers have outlawed several such substances, but the council warned that producers were sidestepping the bans by tweaking the composition of drugs.
It backed creating a new system of broader bans in which all substances chemically similar to controlled drugs were automatically made illegal.
The council also called for suppliers to have to demonstrate that legal highs were not being produced for human consumption and for a fresh drive to alert the public to their dangers.
Roger Howard, chief executive of the Drug Policy Commission, backed the recommendations. "It's increasingly difficult for police to identify the different drugs they are finding," he said.
The Home Office said it was considering the proposals.