You may wonder why teens get addicted to marijuana just as flies get stuck on flypaper, while most adults easily reject the drug. Surprisingly, tests on rats revealed that adolescent rats, too, are more likely than adults to get hooked on cannabis; more than that, the harmful secondary effects inflicted by the drug on their brain (reflected on memory skills) are more severe in their case.

During a test period of 18 days, the team led by Iain McGregor at the University of Sydney, Australia, injected rats with tetrahydrocannabinol (THC), the main active chemical in cannabis, at doses imitating the heavy cannabis use in humans. "Two weeks after the final dose, the adult rats avoided the regions of the testing chamber where they had received injections, but the adolescents showed no such aversion. This suggests that the adult rats found the THC unpleasant, while the adolescents didn't," said McGregor.

And unlike adults, the teen rats displayed lasting problems with short-term memory. When the team investigated the brain's hippocampus area, the researchers discovered changes in many more proteins in the brains of the teenage rats, compared to those of the adults.

The research team suggests that because the teen brain is still maturing, it appears to be more vulnerable to THC. This is a worrying discovery, as figures from the US Substance Abuse and Mental Health Services Administration clearly show that cannabis use among American teens is booming.

This research results are underlined by some recent studies that point to the negative effects of long-term cannabis smoking, from those similar to tobacco smoking (lung conditions, including cancer, and an older look), to severe psychotic illnesses like schizophrenia and brain changes in unborn children (especially in the cortex area, the brain part involved in thinking, learning, attention and planning).

Meanwhile, drug use is strongly supported by many groups as a pain reliever in cases of cancer or AIDS.