A team of leading scientists have spent two years analysing the effects of 20 of Britain's most widely used drugs, and have devised a scientifically rigorous - and controversial - new ranking for them.
The BBC's Horizon programme has followed their research and looks at the science behind the headlines.
How do the brain and body react to each stimulant as it passes into the bloodstream, and what are the long-term effects of drugs relative to their classification?
If the current classification should be changed in view of the latest findings, what exactly is Britain's most dangerous drug?
Watch the full documentary now