The long-running debate over the dangers of cannabis will be reignited by a study that challenges the idea that experimenting with the drug is harmless and does not lead to further drug use.

Prof Yasmin Hurd, Dr Sabrina Spano and Dr Maria Ellgren, working at the Karolinksa Institute, in Sweden, have demonstrated that cannabis can enhance future sensitivity to heroin.

Studying events in the brain of adolescent rats after cannabis exposure, they found that the drug affects the brain's natural chemicals, called endogenous opioids. The chemicals are known to play a role in heightening positive emotions and creating a sense of reward.

That is the same system that is stimulated by hard drugs and is also present in humans. In the case of the rats, those exposed to cannabis as adolescents took more heroin when given the opportunity.

Prof Hurd, who now works at the Mount Sinai School of Medicine, New York, says that, remarkably, although scientists have studied how hard drugs change the brain, similar experiments have not been carried out until now to study the long-term effects of "softer" drugs such as cannabis.

Cannabis is the drug most commonly used by teenagers worldwide, typically by one in five adolescents in America and various European countries during any given month.

This year an independent report commissioned for the Commons science and technology committee concluded that the "gateway" theory - that its use leads on to the use of harder drugs - "has little evidence to support it, despite copious research".

The Swedish team's results show that the brain may "remember" previous cannabis usage and make users vulnerable to harder drugs later in life, specifically opioids such as heroin and morphine.