Physical differences in the brain may increase the chances of a person choosing to take drugs, say Cambridge University scientists.
The make-up of someone's brain 'might predict if they will become an addict'
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.
Up to 500,000 people are currently addicted to Class A drugs such as cocaine, heroin and amphetamines, according to government figures.
One of the most important questions in the science of addiction surrounds the origin of differences noticed in the brains of human drug users.
While these differences are thought to be important in the way humans respond to drugs, it is difficult to prove whether they are a part of the natural brain chemistry of that individual, or have developed as a result of taking the drugs themselves.
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.
When the 'impulsive' rats were introduced to the drugs, and given the opportunity to take them, they were much more likely to do so than the rats with more dopamine receptors.
Dr Jeff Dalley, who led the study funded by the Medical Research Council and the Wellcome Trust, said that this showed clearly that the brain differences, and the impulsivity linked to them, pre-dated any exposure to drugs, with the possibility that the situation in human drug users could be the same.
"What we are talking about here is a possible physical trait producing vulnerability to drug use.
"The next step is identifying the gene or genes that cause this diminished supply of brain receptors.
"This may provide important new leads in the search for improved therapies for attention deficit/hyperactivity disorder (ADHD) and compulsive brain disorders such as drug addiction and pathological gambling."
But he said that the reasons for humans becoming addicted to any drug were more complex than simply their genetic make-up, and that a test for any gene uncovered by further research would not necessarily work.
"There are lots of reasons unconnected to genes why people use drugs, and I can't see that any test would be useful."
Lesley King-Lewis, Chief Executive of Action on Addiction, said: "It is well known that some personality traits are associated with a vulnerability to cocaine and other addiction problems.
"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."
Dr Gerome Breen, from the Institute of Psychiatry in London, said that the differences found in the rats were very likely to have their equivalent in the human brain.
"This is a very exciting study which has successfully identified the biological basis of some of the behaviours that we know are associated with higher risk of cocaine and other addictions in humans.
"It also pinpoints a potential cause of relapse in abstinent drug users - what makes them start taking a drug again despite all the problems they know it will cause them.
"This means that we can start to investigate treatments that, at least partially, correct this deficit in the hope that these will prove successful in preventing relapse."