New research has revealed the level of the active ingredient in party pills varies dramatically from drug to drug.
A study published in the Medical Journal shows big variations in the amount of BZP and TFMPP they contain.
They also found major variations between brands in the time it takes the drugs to 'release' into a user's system.
Researchers say lack of quality control may have led to the toxic effects reported by users unaware of their true content and the time they take to take effect.
They say more stringent regulations are essential to reduce harm.