London, England (CNN) -- An experimental vaccine for cocaine addicts can help some users kick the habit, according to a new study.
An experimental vaccine makes recipients produce antibodies to cocaine.
Researchers at Yale School of Medicine have tested a vaccine that makes people produce antibodies to cocaine.
The theory is that high levels of anti-cocaine antibodies can inactivate cocaine before it enters the brain, preventing users from getting high.
Dr Thomas Kosten worked on the study at Yale, before moving to Baylor College of Medicine, in Texas. He told CNN that this is the first time a cocaine vaccine has been tested in people, and that the vaccine approach would be suitable for all kinds of addictive substances, except alcohol.
"Antibodies are very large proteins and when the cocaine attaches to those antibodies it can't get into the brain or heart, so people don't get high from it," Kosten told CNN.
Because users don't get high, the thinking is that they will eventually stop taking cocaine.
"What was surprising was that we got patients who had limited motivation for stopping cocaine use to basically just run out of money," said Kosten.
Participants received five vaccinations over a 24-week period and their urine was tested for cocaine three times a week.
The study, published in the journal of the American Medical Association, found that the vaccine only achieved the desired antibody level of at least 43 micrograms per milliliter in 38 per cent of subjects, and that level was maintained for two months of the study period.
Of the subjects who achieved desired antibody levels, 53 per cent reduced their cocaine use by half, compared to 23 per cent of those with low antibody levels, and significantly more produced cocaine-free urine samples between weeks nine and 16 of the 24-week study than those who were given a placebo, or had low antibody levels.
But Kosten told CNN that 75 per cent of subjects had high enough antibody levels to block one or two doses of cocaine, and that should be enough to prevent occasional lapses leading back to addiction.
Icro Maremmani, professor of addiction medicine at the University of Pisa and University of Siena, told CNN he believes the vaccine will be useful, but only for highly motivated patients.
"Hard users of cocaine probably will not accept this kind of treatment," he said.
"The situation risks being similar to the use of naltrexone for heroin addicts -- a perfect drug but poor clinical results."
Maremmani said there is also a risk that some patients will increase their intake of cocaine to overcome the antibody effect.
Kosten says it usually takes about two years for addicts to become drug-free, and that patients would have to be vaccinated every three months for that period to maintain antibody levels.
He says that while the vaccine can stop relapses leading back to addiction, it doesn't solve the issues that lead to drug use.
"The vaccine doesn't get you new friends, change your depression, get you out of jail, or all the other things that people have trouble with. People could conceivably be on it for the rest of their life if they can't get it together."