Marijuana varieties are displayed in West Salem Cannabis, a shop in Salem, Ore., on Sept. 27. (Andrew Selsky/AP)

Nearly 40 percent of people who use marijuana say they do so primarily to help them relax. New research from the University of Illinois at Chicago suggests that many of those folks may be overdoing it.

As it turns out, there hasn't actually been a whole lot of research done on the stress-mitigating effects of pot, according to Emma Childs, Joseph A. Lutz and Harriet de Wit of UIC. So they conducted a randomized, double-blind lab experiment involving 42 test subjects to see how different doses of THC — pot's main chemical ingredient — affected the subjects' responses to stressful situations.

The test subjects were divided into three groups. The first group received an oral dose of 12.5 milligrams of THC. The second group got only 7.5 mg, while the remaining students unknowingly received a placebo.

For reference, this is not a lot of THC. With the help of medical researchers, the publication the Cannabist recently calculated that a person smoking half a joint of weed could be expected to consume between 33 and 39 milligrams of THC. So the amounts of THC involved in this study are considerably lower than what a heavy user could conceivably consume in one setting.

Next, the fun part: The researchers placed each of the participants in a high-stress situation. They had to give a five-minute speech, followed by a five minute oral test of subtraction skills before two interviewers. For the final twist of an anxiety-inducing knife, the researchers videotaped the subjects while this was all going on, and included a video display of the interview in the room where it was happening.

The researchers then measured how well the subjects performed in the speech and arithmetic portions, and asked them to subjectively rate how stressful the experience was for them.

The burning question: Did the subjects who dosed on THC perform better in the tests and report less anxiety? And was there any difference in performance between the low and high doses?

They found that compared to the placebo, the low dose of THC “reduced the duration of negative emotional responses to acute psychosocial stress, and participants’ post-task appraisals of how threatening and challenging they found the stressor,” according to the paper.

But the higher dose of THC actually made things worse. “In contrast, the higher dose of THC (12.5mg) produced small but significant increases in anxiety, negative mood and subjective distress at baseline before the tasks began,” and throughout the tasks, the researchers found.

Conclusions? A low dose of marijuana is the way to go if you use it primarily for relaxation purposes.

There are, of course, some big caveats with this. The first is that the subjects ingested the THC orally. The interplay between THC and anxiety may work differently for smoked cannabis.

More significantly, the subjects received a dose of THC alone. The cannabis plant contains dozens of other psychoactive chemicals, some of which are known to produce anxiety-lessening effects on their own. Popping a THC pill in a lab is a very different experience than smoking or consuming whole-plant cannabis.

That said, much of the commercial marijuana market is fixated on producing strains of the plant with the highest possible concentration of THC, often to the exclusion of the other compounds that might mitigate some of THC's negative effects. So heavy smokers, particularly those looking for the biggest buzz, would do well to heed the warnings in the UIC study.