Study: Wait 4.5 hours to drive after getting high

A new study published Jan. 26 shows marijuana-impaired drivers can’t hold the road well for at least 4.5 hours after ingesting a joint.

As a big old stoner, I believe it. I sold my car when I received my medical marijuana card. I used to sit and wait for stop signs to turn green while driving high.

The study appears in the Journal of the American Medical Association. It warns that most people think they are OK to drive after 90 minutes, even though the simulators showed otherwise.

“Bulk cannabis plant material containing 5.9 percent THC, 13.4 percent THC, or placebo was acquired from the National Institute on Drug Abuse Drug Supply Program and hand rolled into 700-mg cigarettes,” according to the study. Participants were told to “smoke the cigarette the way you do at home to get high. You may take up to 10 minutes.” A minimum of 4 puffs was required.

“While the THC group generally reported feeling impaired and hesitant to drive at 30 minutes, at 1 hour 30 minutes participants increasingly rated themselves as safe to drive, whereas simulator data indicated ongoing reduced driving performance, including being more likely to leave their lane,” the study reports. “These first few hours may constitute a period of greatest risk because users who are self-evaluating whether it is safe to drive may be less likely to refrain from driving or to attempt to compensate for reduced functioning.

“This is an important topic for public safety messaging, since a goal is to keep impaired drivers off of the road prior to becoming a danger.”

No correlation between blood THC levels, driving performance

However, there was no correlation between blood THC levels and driving performance. That is, people with higher levels of THC in their blood did not necessarily perform worse than those with a small amount of THC in their blood.

Some of you likely are saying, “My tolerance is so high, 10 minutes of puffing on a joint barely effects me.” But the study proved otherwise. “Greater intensity of cannabis use in the past six months was associated with reaching higher blood THC concentrations following smoking but not self-reported greater levels of highness nor worse driving performance than lower-intensity groups, consistent with development of behavioral tolerance,” according to the study.

“However, the current findings also suggest that when instructed to achieve a self-determined level of highness, users with a history of greater use intensity adapted to tolerance by increasing THC exposure, resulting in performance decrements similar to users with lower-intensity use.”

Those with high tolerance consume more, still get high

Bottom line? Old stoners who top their flower with concentrate get high like first-time smokers and may not be less of a driving risk. “Behavioral tolerance benefits may be more apparent in medicinal users who target specific symptoms (e.g., pain) and maintain a consistent dosing level.”

Curiously, cannabis’s effect on people varies widely. It’s not like alcohol, where driving impairment rises with intake. “Although performance was improving at 3.5 hours, recovery was not fully seen until 4.5 hours post-smoking,” the study concluded. “The fact that not all participants consuming THC met the criteria for impairment underscores the interindividual variability seen with the impairing effects of cannabis. The lack of relationship between blood THC concentration and driving performance raises questions about the validity of per se laws.

“When users control their own intake, one cannot infer the level of impairment based on the THC content of the product, the level of behavioral tolerance in the individual, or the blood THC concentration. Future research should address factors such as individual biologic differences, personal experience with cannabis, and cannabis administration methods in relation to driving impairment.”