Scientists understood why people get addicted to a class of prescription drugs that includes Valium, along with other antidepressants, muscle relaxants and sleeping pills.
The findings, reported in the British science journal Nature, open a path to developing new treatments that can ease symptoms without causing dependence, the researchers hope.
Opium, heroin and cannabinoids are addictive because they activate a neural circuit that boosts levels of a reward-giving brain chemical called dopamine.
The molecular pathway they use to unleash the dopamine is well known.
Benzodiazepines -- which include Valium, Xanax, Librium, Ambien and other well-known drugs -- likewise stimulate the dopamine system, but until now how they did this was unclear.
In a series of experiments on mice, a team led by Christian Luscher at the University of Geneva say they have solved the mystery.
Benzodiazepines switch on a intermediary neurotransmitter known as gamma-aminobutyric acid, or GABA, increase dopamine levels, resulting in the signature craving of addiction, they found.
The pathway that has been uncovered does not have an effect on benzodiazepines' therapeutic effects.
As a result, it should be possible to design new drugs that deliver the same benefits but without inducing addiction, said the researchers.
The work "is a landmark for the field," noted Arthur Riegel and Peter Kalivas of the Medical University of South Carolina in Charleston.
"These authors are the first to identify a molecular mechanism contributing to benzodiazepine abuse," the pair wrote in a commentary, also in Nature.