The scientists trying to develop a vaccine to prevent heroin addiction are almost ready to begin human clinical trials, Mexico’s Juan Ramon de la Fuente National Psychiatric Institute said.

Tests of the medication on animals have achieved “significant advances related to the time in which the subjects remain immunized,” the institute’s head of epidemiological and psychosocial research, Dr. Maria Elena Medina-Mora, told a press conference here.

Rats that became addicted to heroin after being allowed to ingest as much of the drug as they wanted began to consume less of it once they received the vaccine, she said, describing the results as “very encouraging.”

The vaccine blocks the pleasure-inducing effect of heroin by preventing the drug from breaching the blood-brain barrier, Medina-Mora said.

If the medication works on humans, she said, it “will be administered to people who have a serious addiction.”

The institute’s 15-year-long effort to develop the vaccine was financed largely by the Mexican government and received technical support from the U.S. National Institute on Drug Abuse, she said.

Mexico’s health ministry announced earlier this month that it had obtained a U.S. patent for the anti-addiction medication.