Scientists say they now know more about what happens in the brains of former drug addicts when they crave cocaine.
The research could help understand the addition process
Tests on rats by US National Institute on Drug Abuse researchers found a way of blocking craving messages in their brains and preventing a relapse.
Nature Neuroscience says the findings suggest the danger time is not straight after addicts stop taking the drug, but after they go long periods without it.
The study found a protein in the brain plays a part in triggering cravings.
In their study, the team trained rats to press a lever to receive an intravenous injection of cocaine.
The rats were also trained to associate certain cues, such as a light, with the availability of the drug.
Once this behaviour pattern was established, the researchers withheld both cocaine and cues associated with the drug from the rats for a month.
When the rats were again exposed to the cues after 30 days of withdrawal, they showed much higher levels of craving than they did after just one day.
When the researchers examined the rats, they looked at the activation of a protein called extracellular-signal regulated kinase (ERK) in the amygdala, a part of the brain involved in motivation and emotion.
ERK is also known to play a role in learned fear, where someone can feel frightened even when they only see or hear something that they connect with the actual fear trigger, rather than the trigger itself.
This study found ERK activation in the amygdala was higher 30 days after withdrawal than one day after withdrawal.
The researchers also found that blocking ERK activation decreased cocaine-seeking behaviour in the rats, while activating ERK one day after withdrawal increased the rats' cocaine-seeking behaviour.
The researchers say they have identified a specific biochemical pathway in the amygdala which underlies latent cocaine cravings which, they add, gives an insight into the mechanisms of drug relapse.
Martin Barnes, chief executive of DrugScope, said: "This research is interesting, but we have to be careful about drawing too direct a comparison with the nature of human addiction.
"Making the obvious point, there is little correlation between the brain patterns and behaviour of mice and humans."
He said relapse was often linked to external social factors such as lack of housing and unemployment.