Physical differences in the brain may increase the chances of a person choosing to take drugs, say Cambridge University scientists.
A study of rats showed variations in brain structure pre-dated their first exposure to narcotics, and made them more likely to opt for cocaine.
Writing in Science, the team say genes may affect these differences in humans.
Treatments to reduce their effect may be found - but a test of vulnerability to drugs is unlikely, they add.
To unravel this problem, the Cambridge researchers scanned the brains of rats, and found similar differences in 'neurotransmitter receptors' in certain parts of the brain.
Some of the animals had far fewer 'dopamine receptors' - the brain structures onto which drugs such as cocaine and heroin latch to produce their effect.
The scientists used a game in which the rats had to wait to press a button and receive a reward, coupled with detailed brain scans, to see if those with the fewest dopamine receptors were impulsive, a type of behaviour often linked with drug use in humans.
This was the case - even in rats which had no contact with drugs.
"This study is extremely interesting because it has identified a biological basis in rats for some of the behaviours that we know are associated and shows how they can lead to drug addiction."
