THC Levels May Not Correlate with Impairment

THC Impairment
Research on the effects of marijuana has been limited due to federal restrictions, but with the majority of states now permitting medical cannabis use, efforts are underway to understand its short and long-term effects. A study by the Journal of the American Medical Association (JAMA) Psychiatry suggests that higher THC levels may not necessarily correlate with greater impairment. The study found that despite higher THC exposure, users of cannabis concentrates did not show greater short-term subjective, cognitive, or balance impairment compared to cannabis flower users. This calls into question the use of THC blood tests to determine intoxication and suggests that balance impairment could be a more useful marker of recent cannabis use.
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Due to the federal restrictions on cannabis use, research about marijuana’s safety and effects has been limited. Now that the majority of states permit medical cannabis use (and eleven even allow recreational use), efforts are underway to understand both the short and long-term effects of marijuana on neurocognitive and behavioral intoxication.

Because the potency of marijuana products is increasing, researchers with the Journal of the American Medical Association (JAMA) Psychiatry designed a study to measure the neurobehavioral effects of differing concentrations of delta-9-tetrahydrocannabinol (THC) in the flower form of marijuana in comparison to concentrated forms. (Cannabis concentrates are made by extracting the plant’s cannabinoids to produce a much higher THC concentration, often in the 80% range).

And, while popular opinion holds that “more THC equals more intoxication” – with several scientific reviews strengthening the claim – this new study sheds a provocative light on the subject, suggesting that THC might not be the deciding factor in marijuana impairment after all.

What Is Marijuana Impairment?

Numerous studies have highlighted the adverse neurobehavioral effects of cannabis intoxication, including both cognitive and motor effects. These include:

  • Verbal recall (memory)
  • Selective attention
  • Inhibitory motor control
  • Coordinated movement (balance)

Measuring marijuana intoxication has garnered increased media attention as police seek ways to monitor marijuana-impaired drivers in legalized cannabis states. To date, blood tests indicating THC concentration have been the go-to. But the JAMA Psychiatry study below may just flip that approach on its head.

How Was the Study Conducted?

Out of 133 initial participants, 121 people (55 flower; 66 concentrate) completed the trial as instructed. Flower users were randomly assigned to flower containing either 16% or 24% THC concentration, while concentrate users were randomized to 70% or 90% THC concentrates.

The primary focus of the study was to measure the level of THC and THC by-products in the blood, the level of subjective drug intoxication, and neurobehavioral testing for attention, memory, inhibitory control, and balance.

Specific questions addressed in the study were:

1.    How does the short-term use of the cannabis flower and concentrates correlate with THC plasma levels, subjective intoxication and mood, cognitive performance, and balance?

2.    Do these four associations differ between flower and concentrate users?

3.    Do these four associations differ by THC potency for each form of cannabis (16% vs. 24% flower or 70% vs. 90% concentrate)?

Research Results

Researchers uncovered a number of interesting results. For instance, the concentrate group exhibited consistently higher THC and 11-OH-THC levels in their blood after short-term use, but that level did not vary much between the 70% and 90% potency groups.

Concerning mood, all participants reported experiencing little to no tension, little to moderate vigor, and moderate to marked intoxication. The concentrate group reported lower tension and vigor compared to the flower group, regardless of potency.

The flower group also performed better concerning inhibitory control across all assessments, and there were no short-term changes in inhibitory control after use or an association with potency.

Researchers also made several generalized observations regarding marijuana impairment:

  • Standing with eyes open showed impaired balance (a sway of approximately 11%) after cannabis use that normalized within an hour
  • Standing with eyes closed was associated with sway in the short-term that increased post-use and decreased back to pre-use levels one hour after use
  • Standing with eyes closed-head back showed no change from pre-use to post-use even though there was poor performance overall

But perhaps most importantly, the results of this study indicate that the use of cannabis for medical reasons is safe with short-term reversible neurobehavioral effects:

In general, across most cognitive measures, acute performance changes following cannabis use were minimal.

Why THC May Not Be the Culprit

This study is the first to address and compare the association of self-administered legal cannabis flower and concentrates on subjective intoxication and mood, cognition, and balance in people who are regular cannabis users.

However, in comparing the two routes of administration, researchers uncovered an arguably more important discovery:

Compared with cannabis flower users, cannabis concentrate users had higher plasma levels of THC.

However, despite this higher THC exposure, concentrate users did not show greater short-term subjective, cognitive, or balance impairment.

This suggests that THC plasma levels and impairment may not be so inextricably linked –  calling into question law enforcement’s use of THC blood tests to determine intoxication.

But why didn’t increased THC levels correlate with impairment in this study?

One theory is that both groups likely self-titrated to achieve similar exposures despite higher potency. Another possibility is that concentrate users had a higher tolerance level for THC due to regular use. One other possibility is that participants’ cannabinoid receptors may have become saturated, thus diminishing the effect of more THC. Finally, the results may be explained by individual genetics or biological sensitivities affecting the metabolism of THC.

Ideas for Testing Marijuana Impairment

According to the present study, the degree of balance impairment found with THC is similar to the balance effects of moderate alcohol (0.05%-0.10% blood alcohol concentration compared to lower-potency THC administration in regular users).

As researchers point out, this finding offers an interesting alternative to THC-focused impairment tests:

Balance impairment could be a useful marker of recent cannabis use, even in regular users of highly potent products who show tolerance to short-term cannabis use in other domains.

So how will these study results influence driving and employment drug tests? Only time will tell. But as the present research suggests:

The scientific community has taken the first step in walking away from the “more THC equals more impairment” theory.

Updated: July 27, 2023

Article Written By:

Kimberly Langdon

Kimberly is a professional writer specializing in medical cannabis and alternative health reporting.


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