Researchers in Britain have found that abnormalities in the brain may make some people more likely to become drug addicts.
Experts were previously unsure whether it was the drugs that altered the wiring of the brain, or if brains were wired differently in the first place.
Now scientists at the University of Cambridge have found the same differences in the brains of addicts and their non-addicted brothers and sisters.
Their findings have been published in the journal Science.
The BBC's science editor says it has long been established that the brains of drug addicts have some differences to other people, but explaining that finding has been more difficult.
The brains of 50 cocaine or crack addicts were compared with the brain of their brother or sister, who had always been clean.
Both the addicts and the non-addict siblings had the same abnormalities in the region of the brain which controls behaviour, the fronto-striatal systems.
The suggestion is that these brains may be "hard-wired" for addiction in the first place.
The study was funded by the Medical Research Council,