People who begin drug use by smoking marijuana may be at increased risk for nicotine addiction when they later try cigarettes, a new study in mice suggests.
In the study, researchers first exposed mice to tetrahydrocannabinol (THC, the active ingredient in marijuana) or a placebo for three days. Then, they attempted to train the mice to self-administer nicotine, which was delivered into their blood when they poked their noses through a hole in the cage.
Mice don't always learn how to do this. Presumably, those who successfully learn how to get their own nicotine are more motivated to get a fix — in other words, they are more "addicted."
Among the mice exposed to THC, 94 percent learned how to self-administer nicotine, compared with 65 percent of mice who'd been exposed to the placebo. In addition, THC-exposed mice were willing to work harder to get nicotine — performing up to 17 "nose-pokes" for a dose of nicotine, compared to 11 nose-pokes in the placebo group.
It's more common for people to smoke cigarettes before trying marijuana (and in this case, cigarettes are considered a "gateway" drug). But in a substantial number of cases, people use cannabis before progressing to cigarettes, said the researchers from the National Institute on Drug Abuse.
The new findings suggest "a history of cannabis exposure might have lasting effects that increase the risk of becoming addicted to nicotine," the researchers wrote in the Feb. 6 issue of the journal Neuropsychopharmacology.
However, because the study was conducted in mice, it's not clear whether the findings translate to people. In addition, there are likely other factors that could contribute to the progression from marijuana to other drugs, including a genetic susceptibility to addiction, and social factors.
Interestingly, in previous studies, THC-exposure did not increase the animals' risk of acquiring an addiction to heroin or cocaine. It's possible the brain systems responsible for feelings of reward in nicotine and marijuana users interact in some way, the researchers said.