A new study published in the British Journal of Psychiatry suggests that variations in the chemical makeup of different strains of marijuana are associated with different levels of cognitive impairment while high.

Tetrahydocannabinol (THC) is commonly recognized as the ingredient in marijuana that causes a “high” in users, but researchers have long known that pot contains other active substances as well. While THC can cause the widely advertised hallucinations and paranoia, another chemical found in marijuana, cannabidiol (CBD), is believed to be responsible for the calmer, sedating part of the experience. The two chemicals have opposing effects on one of the brain receptors affected by cannabis, the CB1 receptor.

To determine the effect of different levels of CBD, researchers studied 134 cannabis-using volunteers while they smoked their own stash of marijuana, at home. They gave them various cognitive tests, either while stoned or abstinent. Then, they took samples of the pot back to the lab for testing.

The amount of cannabidiol contained in the marijuana varied widely – from 0.14% or less to 0.75%. Researchers found that individuals that smoked the weed with the lowest CBD had significant impairments in their ability to recall words, while those whose pot had higher levels of the chemical had no impairment at all. (The study authors controlled for any variance in levels of THC.)

A previous study had not found this difference – but the author of that research told Nature News that he thinks this is because his group studied only the marijuana supplied by the U.S. government, rather than the much more varied weed smoked by illegal users.

Another variable might be how often the users typically smoke: a recent study of frequent marijuana users found that while cannabis did slow reaction time, it didn’t have a significant effect on cognitive performance.

Interestingly, however, unlike previous studies, the new research did not find that CBD reduced the hallucinatory and paranoia-inducing effects of THC that can be associated with psychotic episodes.

While some growers have bred “skunk” marijuana, which has extremely high levels of THC, less intense varietals with increased CBD can also be found. If this research holds up, the mellower high may be the smarter choice.