"The study shows that the modern cigarette does to nicotine what crack does to cocaine," says addiction expert Jack Henningfield, at the Johns Hopkins University School of Medicine. The notorious addictiveness of smoking crack results from the vapourised cocaine reaching the brain almost immediately.

Ian Jones, a nicotine expert at Bath University, UK, adds: "Free-base nicotine is the most damaging form because it is the optimal configuration for binding to the nicotine receptors in the brain, heart and rest of the body. If the binding efficiency is increased, it means the concentration of nicotine at the receptors is higher and so it is very addictive."

New Scientist 28 July 03