Rats given doses of the active ingredient in marijuana early in life were more sensitive to other drugs later in life, according to Swedish researchers.

Researchers led by Yasmin Hurd of the Karolinska Institute exposed young rats to THC, the main psychoactive ingredient in marijuana, then studied their response to heroin use. The THC-exposed rats tended to be more responsive to lower doses of heroin and self-administer more of the drug than the non-exposed rats.

The study could have implications for teenage humans who use marijuana, perhaps making them more susceptible to other drug use later on, researchers said. “The developing brain is definitely more sensitive,” Hurd said, noting that the human brain continues to develop up to age 25. “Many people think that all cannabis does is to give you a calm, relaxed feeling and no long-term effects.”

Commenting on the study, professor Robin Murray of the Institute of Psychiatry in London said, “Clearly it needs to be replicated, but there is already evidence that, in animals, cannabis and amphetamine show cross-tolerance. So that rodents given THC, the active ingredient of cannabis, show greater effects when given amphetamine. This suggests that it may be easier to come to grief when you try heavy drugs if you have already sensitized your brain receptors with cannabis. We need more basic and clinical research into the long-term effects.”

Source: Daily Telegraph. June 2005