How high is too high to drive?

With cannabis legalization around the corner, here's a summary of the research that you can reference when your patients ask



It is only a matter of time until one of my patients asked this question:

“Dr. Beck, how much marijuana will I be able to use and still drive?”

“I didn’t think you were driving yet.”

“I’m not driving officially, but I’m going to get my G1.”

“So, you don’t even have a G1 and you’ll be too young to buy marijuana legally, but you want to know how much you can use and still be able to drive. Is that what you’re asking me?”

“Well, when you say it that way, it makes it sound like I shouldn’t consider this at all, but I’d like to know.”

In fairness to this young person, I should say that both youth and parents are asking me similar questions, so I decided to see what I could learn about driving under the influence of cannabis. For the purposes of preparing this short review, I used three main references. One reference is a report entitled Developing Science-Based Per-Se Limits for Driving under the Influence of Cannabis: Findings and Recommendations of an Expert Panel. The report was written in 2005 and includes a thorough summary of the empirical research on cannabis and driving. I checked with the medical librarian at my hospital to see whether they could find a more recent and similarly comprehensive report. Since they could not, I highly recommend this excellent reference.

I also reviewed the research of the American Automobile Association and the Canadian Automobile Association which, while not as comprehensive, is much more accessible. I use these as references for most people who ask me for information.

Finally, I regularly review the Government of Canada and the Government of Ontario websites for updates on the legalization of cannabis. The Government of Ontario has determined that the Ontario Provincial Police will use oral fluid screening devices for roadside testing of cannabis consumption. There are some concerns regarding the reliability of the devices and, since the expert panel report recommended blood levels only as a means of measuring cannabis consumption (rather than impairment), it seems that cannabis will be legalized without clear guidelines regarding impairment comparable to blood alcohol levels.

When discussing driving safety while using any substances, I always stress that a person should never drive if they feel at all affected by what they have ingested. As most people know, the extent to which a person can feel affected by a substance they ingest can vary widely from person to person, but if an individual is being honest with themselves, this can be a very reasonable guideline.

For those wanting something more precise related to cannabis use, I did determine a straightforward guideline from the information in the expert panel report. The 2005 report reviewed the results of all the epidemiological studies related to driving and cannabis use and performed a meta-analysis of the experimental studies.

In reviewing all the studies, the panel explained that there are three phases of impairment due to cannabis use. There is an acute phase, which is the first 60 minutes following consumption. Then there is a post-acute phase that presents 60-150 minutes after use. There is, finally, a residual phase, occurring 150 minutes after ingestion. The panel members concluded that most studies showed that the impact on driving skills is minimal during the residual phase. This means that, depending on the amount of cannabis smoked, most people will no longer be affected 3-4 hours after smoking 20 mg of cannabis.

This is the basis of the guideline I am providing: Don’t drive for three to four hours after smoking cannabis, depending on the dose. (Note: The studies also showed that there are differences in the timing of impairment after smoking cannabis, as compared with oral ingestion. Impairment relation to oral ingestion of cannabis peaks two to three hours after consumption, while impairment after smoking cannabis peaks at one hour.)

I like this guideline because it is clear and straightforward, although I am also reminding the young adults in my practice that most of them would never go out for an evening during which they might drink at all without a designated driver. My own practice suggests that, as far as alcohol is concerned, most youth I know would never drink and drive. Why not also have the same designated driver system when using cannabis?

As I review these studies and reports in conjunction with the information being provided by the federal government, I continue to be concerned about the lack of foresight in legalizing cannabis in Canada. It seems to me that the amount of time needed to ensure the safe legalization of cannabis might have been more accurately predicted. If the timing was not well-managed, can we really be sure that other aspects of legalization will be handled safely?

Dr. Gail Beck is a child psychiatrist in Ottawa. She serves on the executive committee at the Medical Women’s International Association, and is a past board member at the Canadian Medical Association.

Opinions expressed in this article are those of the writer, and do not necessarily reflect those of or its parent company.