Study finds stronger marijuana doesn’t lead to higher levels of intoxication

A new study led by a team from the University of Colorado, Boulder, has found smoking high-THC marijuana or concentrates does not result in greater levels of intoxication. The research adds weight to a growing body of evidence suggesting proposed roadside THC tests may not correlate specifically with driving impairment.

Using mobile laboratories, dubbed "cannavans" by the researchers, 121 regular cannabis users were assessed after smoking either a THC concentrate or a marijuana flower. The flower smokers consumed product containing either 16 or 24 percent THC, while the concentrate users compared 70- and 90-percent THC products.

All subjects had blood taken at three time points during the experiment – before smoking cannabis, immediately following consumption, and one hour later. Subjects also self reported intoxication levels and conducted cognitive function and balance tests at all three time points.

“Surprisingly, we found that potency did not track with intoxication levels,” says Cinnamon Bidwell, lead author on the new study. “While we saw striking differences in blood levels between the two groups, they were similarly impaired.”

Those consuming the high-potency concentrate registered significantly high blood THC levels in the minutes after smoking, on average more than 1,000 micrograms per milliliter compared to those smoking the flower, averaging around 455 micrograms per milliliter. Strikingly, all the subjects reported similar subjective feelings of intoxication and achieved similar results on balance and cognitive function tests, regardless of what kind of cannabis they consumed or how strong their product of choice was.

“People in the high concentration group were much less compromised than we thought they were going to be,” explains Kent Hutchison, co-author on the study. “If we gave people that high a concentration of alcohol it would have been a different story.”

The absence of any correlation between THC blood levels and intoxication surprised the researchers. Alcohol, for example, provides a distinct association between blood concentration levels and impairment. The more alcohol one drinks, the higher their blood alcohol level, and the more visibly intoxicated they become. However, THC doesn’t seem to offer the same reliable impairment curve, and the study hypothesizes several reasons why this may be the case.

Tolerance to the effects of THC is the most obvious explanation, but the study suggests tolerance does not explain the entirety of the mystery. “Another possibility,” the researchers hypothesize in the published paper, “is that cannabinoid receptors may become saturated with THC (a cannabinoid receptor 1 partial agonist) at higher levels, beyond which there is a diminishing effect of additional THC.”

The huge discordancy between THC blood levels and subjective impairment means it will be challenging for authorities to roll out some kind of roadside THC test. While blood alcohol is easily measured through a breathalyzer, and higher levels comfortably correlate with impairment, this study affirms prior research suggesting THC blood levels can vary wildly from person to person and do not consistently correlate with impairment.

Interestingly, a unique balance test was conducted by the researchers as part of this study, and a small degree of balance impairment turned out to be the most consistent short-term sign of performance impairment across all the subjects. An 11-percent increase in sway, during an eyes-closed balance test, was seen immediately following cannabis consumption across all subjects. This balance impairment returned to normal by the one-hour follow-up suggesting this kind of balance test may be a useful way to measure cannabis intoxication, similar to the field sobriety tests currently used to identify alcohol-impaired drivers.

Hutchison does point out the study raises several novel questions that future research will need to address. Particularly, it is unclear what the long-term effect of concentrated high-THC exposure has on regular users.

“Does long-term, concentrated exposure mess with your cannabinoid receptors in a way that could have long-term repercussions? Does it make it harder to quit when you want to?” asks Hutchison. “We just don’t know yet.”

The new study was published in the journal JAMA Psychiatry.

Source: UC Boulder