Taking cannabis as a teenager really does pave the way to heroin addiction in later life, say scientists.

Researchers have found that cannabis acts as a 'gateway' drug, because exposure during adolescence primes the system to crave the chemical stimulation of hard drugs.

Previously it has been argued teenagers move to harder drugs because of peer or emotional pressures, or easy availability.

The study, published online today in the journal Neuropsychopharmacology, looked at the behaviour of young rats exposed to cannabis. These rats took in much larger doses of opium when trained to self administer than other rodents.

Dr Yasmin Hurd of the Mount Sinai School of Medicine, New York, said cannabis affects some of the brain's natural chemicals which heighten positive emotions.

The same system is stimulated by hard drugs meaning the brain may 'remember' previous usage.

The adolescent brain is particularly sensitive to drug exposure, she said, because of cell changes in areas essential for behavioural functions.