More than half of the doses being sold as the illegal drug GHB in nightclubs are actually made from a perfectly legal compound, the BBC has learned.
Drugs groups say GBL is becoming much more widespread
The Tictac drugs database says about 60% of suspected GHB samples are in fact legal gamma butyrlactone (GBL), found in nail varnish remover.
The Advisory Council on the Misuse of Drugs is investigating and is due to report back to the Home Office shortly.
Like GHB, an overdose of highly toxic GBL can lead to coma or death.
GHB, or gamma hydroxybutyrate, occurs naturally in the brain in tiny quantities, but when taken as a drug induces euphoria and reduces inhibitions.
It is a banned class-C drug and anyone found in possession of it faces up to two years in prison and an unlimited fine. Dealing can result in a sentence of up to 14 years.
According to Tictac, more than half of the samples of suspected GHB seized in clubs now contain GBL - an industrial solvent used in the plastics industry and items as diverse as nail varnish removing pads and motorcycle chain cleaner.
GBL turns into GHB once it passes into the bloodstream.
Both are usually found as clear liquids, but GBL is legal to purchase and possess.
John Ramsey, from Tictac, which is based at St George's Hospital in South London, said: "What we actually find in the clubs is a whole range of containers containing liquids... one contains GHB, but there's an identical bottle that contains GBL.
"When we first started collecting data a couple of years ago we were seeing largely GHB with a small amount of GBL, whereas now about 60% of what we are finding is GBL."
Mr Ramsey added: "GBL is completely uncontrolled."
People who use the substances cannot tell the difference between GBL and GHB and both can cause blackouts and other serious problems including suppression of breathing and coma.
Graham Johnson, a consultant in Emergency Medicine at Leeds General Infirmary, says he is seeing between three and five cases each week of poisoning caused by the compounds.
"Some people require intensive care and nationally, there have been deaths associated with abuse of GHB," Mr Johnson said.
GHB abuse started mainly on the gay club scene, but there is some evidence that this has widened out to more mainstream use.
It has also been associated with drug-assisted sexual assault.
Although the evidence for this is relatively limited, Pc Russell Pritchard, from the Metropolitan Police, believes it can make people a target even when self-administered.
"People are intentionally taking significant amounts of GHB or other recreational drugs and are then putting themselves at risk of either sexual assault or robbery," Pc Pritchard said.
The Advisory Council on the Misuse of Drugs could recommend GBL be classified an illegal substance just like GHB.
Alternatively, it could call for tighter voluntary codes for industries who use it or for an unpleasant tasting chemical to be added to it to discourage use and make it easier to identify.
The council's advice is due to be published in the New Year and it will then be up to the government to respond.