James G. Wigmore, Forensic Toxicologist, Speaker, Author

This is the first blog in the new area of drugs that I will be blogging about – cannabis.  It may seem like I have to change the name of the website from Wigmore on Alcohol to Wigmore on Weed (WOW!).  But actually, tetrathydrocannabinol (THC), the main psychoactive compound of marijuana is an alcohol.  That’s right – an alcohol, just like ethanol or methanol.  The structure and metabolism and effects are more complicated than for a simple molecule like alcohol, but it makes THC more interesting in some ways.

The structure of THC is as follows:

THC is a hydrocarbon with just hydrogen, carbon and oxygen, and it is the –OH group which makes it an alcohol.

Cannabis and Driving

The stereotypical view of the effect of smoking cannabis on driving is that the driver becomes more cautious and drives slower than normal and so will not get involved in as many accidents as the alcohol-intoxicated driver. There are a number of studies which shows that aspects of this stereotype is true. In one study, the postmortem blood samples of 372 drivers killed in Norway between 2005 and 2010 were analysed for alcohol and 15 other psychoactive drugs including THC. A driver was considered under the influence of alcohol if the BAC was 0.050 g/100mL or greater. The following Table shows the Odds Ratio (OR) of the fatally injured driver being unbelted, or speeding with alcohol or other drugs:

It is interesting that drivers with THC only were more likely than even sober drivers to have been seat-belted which showed a more cautious behaviour, but were still over 3X as likely to speed than sober drivers.  The most unsafe drivers were those who had consumed alcohol.

I am not advocating smoking marijuana before driving as there is an increased risk of fatal collisions than for sober drivers. The drug and alcohol-free driver is the safest one.  But it appears that the decriminalization of marijuana may not be as great a public safety concern for traffic safety as with alcohol.


The conclusion of the authors of this study was:

Excessive speeding is one of the main reasons for road traffic crashes and together with being unbelted the main reason for a fatal outcome.  This behavioUr might in many cases be due to increased risk taking or negligence of safety measures as a result of alcohol or drug use.


Bogstrand, S.T., Larsson, M., Holtan, A., Staff, T., Vindenes, V., and Gjerde, H., “Associations Between Driving Under the Influence of Alcohol or Drugs and Speeding and Seatbelt Use Among Fatally Injured Drivers in Norway”, Accident, Analysis and Prevention, 78: 14-19, 2015

“Wigmore on Alcohol:  Courtroom Alcohol Toxicology for the Medicolegal Professional”, Chapter 5, The Effect of Alcohol on Driving Ability, Irwin Law Publishing, 559 pp, 2011