Marijuana with relatively high levels of a compound called cannabidiol may be less risky to smoke over the long term, because this ingredient may counteract some of the drug's harmful effects, according to a new study in mice.

The study found that adolescent mice injected with frequent doses of tetrahydrocannabinol (THC) — the ingredient in marijuana that produces a "high" — showed signs of impaired memory and increased anxiety over the long term. But mice that received daily doses of THC combined with an equal amount of cannabidiol (CBD) did not experience these negative effects.

The study "suggests that strains of cannabis with similar levels of CBD and THC would pose significantly less long-term risk due to CBD's protective effect against THC," study author Dr. Ken Mackie, a professor in the Department of Psychological and Brain Sciences at Indiana University, said in a statement.

Over the last several decades, THC levels in marijuana used in the United States have increased 300 percent, while levels of CBD in marijuana have decreased, the researchers said. But the long-term effects of exposure to THC and CBD need to be studied further, they said. [25 Odd Facts About Marijuana]

Studies on whether CBD can counteract the negative effects of THC have been mixed, according to a 2013 review paper on the topic. For example, some studies have found that people experience fewer symptoms of anxiety when they smoke marijuana with high CBD levels, compared to when they smoke marijuana with high THC levels, but other studies failed to replicate these results, according to the review. Another study, published in 2011, found that people who tended to use marijuana products with high CBD levels were at lower risk for psychotic symptoms over the long term, although the effect was subtle.

In the new study, the researchers examined the effects of THC and CBD on both adolescent and adult mice. The mice were divided into five groups: a group that received THC only; a group that received CBD only; a group that received THC and CBD; a group that received a placebo; and a group that received no treatment. The mice in the first four groups were injected with the substances every day for three weeks. The researchers examined the mice shortly after their drug treatment and after a six-week drug-free period.

The researchers found that, immediately after treatment, the mice exposed to THC alone showed signs of impaired memory and increased obsessive-compulsive behavior. Six weeks later, the adolescent mice still showed these symptoms, while the adult mice did not. (This finding agrees with research in humans suggesting that teens may be at greater risk for long-term problems from marijuana, compared with adults.)

However, both the adolescent and adult mice that were exposed to THC experienced long-term increases in anxiety.

In contrast, mice that were exposed to both THC and CBD together (in equal amounts) showed no changes in their behavior, either over the short or long term.

"This is the first study in a rigorously controlled animal model to find that CBD appears to protect the brain against the negative effects of chronic THC," Mackie said.

Animal models allow researchers to conduct experiments in a controlled way, without putting people at risk for harm from the study. But findings in animals don't always translate to humans.

More studies are also needed to determine how CBD counters the effects of THC, and how much CBD is needed to confer a protective effect, the researchers said.

Earlier this year, Canada issued new guidelines for how people can lower their risk of health problems linked to marijuana use, if they choose to use the drug. (In April, Prime Minister Justin Trudeau introduced legislation to legalize the recreational use of marijuana in the country.) One of the recommendations was to choose products with high levels of CBD relative to THC, which the guidelines also said may be "lower- risk" products.

Original article on Live Science.