How Long Must I Wait Before Driving?

Canadians want to know how long they have to wait to drive after smoking a marijuana cigarette

Most people know that, with consumption of alcohol, your blood alcohol concentration builds up with each drink, reaches a plateau after you have stopped drinking, and then the BAC drops at a rate of 10 to 20 mg/100 mls per hour. That means that you can roughly calculate how much you've had to drink, know your body weight, and work out how many hours it will take before your blood alcohol concentration drops below 80 mg/100mls (Canada criminal offence), 50 mg/100mls (Ontario limit), 0 mg/100mls (safe driving). You know that, if you drink in moderation and you are not an alcoholic, your BAC will be at a safe level, sometime the next day.

But it doesn't work that way with THC Cannabis. (Please remember that CBD cannabis will have some THC.) Actual THC impairment rises and falls very quickly. If you smoked one marijuana cigarette awhile ago, it is possible that you are not actually impaired. You probably won't feel impaired and your friends may say you are not impaired. The problem is that isn't how the new DUID law works in Canada. Under Bill C-46, police are being given a very wide discretion to say that you are impaired. They can compel use of SFSTs and DRE evaluations. Police are authorized to compel use of approved drug screening equipment, that uses an immunoassay IA system that merely detects a threshold level of a metabolite; it is not capable of producing a reliable quantitative analysis of THC for a forensic purpose. Let's suppose that an officer conducts a drug recognition evaluation and asks you if you smoke marijuana. You say "yes" and the officer makes a drug recognition expert determination that you are impaired by cannabis. Whether they are right or not isn't the point. You pee in the cup and the urine is sent to a lab for a proper quantitative analysis. Weeks later, the lab says that they didn't find any concentration of THC but they found a trace of a metabolite of THC. When it goes to Court the prosecutor will rely on a presumtion in the new law that says:

Admissibility of evaluating officer’s opinion

(5) An evaluating officer’s opinion relating to the impairment, by a type of drug that they identified, or by a combination of alcohol and that type of drug, of a person’s ability to operate a conveyance is admissible in evidence without qualifying the evaluating officer as an expert.

Presumption — drug

(6) If the analysis of a sample provided under subsection 320.‍28(4) demonstrates that the person has a drug in their body that is of a type that the evaluating officer has identified as impairing the person’s ability to operate a conveyance, that drug — or, if the person has also consumed alcohol, the combination of alcohol and that drug — is presumed, in the absence of evidence to the contrary, to be the drug, or the combination of alcohol and that drug, that was present in the person’s body at the time when the person operated the conveyance and, on proof of the person’s impairment, to have been the cause of that impairment.

Admissibility of result of analysis

(7) The result of an analysis of a sample of a person’s breath, blood, urine, sweat or other bodily substance that they were not required to provide under this Part may be admitted in evidence even if the person was not warned before they provided the sample that they were not required to do so or that the result of the analysis of the sample might be used in evidence.

The problem is that you need to think about what metabolites of THC in cannabis (e.g.  THC-COOH) are still stored in your system a month or more after consumption. Those metabolites may show up in your system on the urine test. They may not show up as THC itself but the existence of those trace metabolites, combined with a police officer's DRE opinion,may be enough to result in a conviction under the new legislation.

The new automated drug screening equipment (currently limited for use as a screening device only, not for quantitative analysis) has not yet been independently tested by scientists in private practice. The defence Bar does not know yet what actual levels, of which metabolites or interferent substances, may trigger false positives.


Here are some compounds that are structurally related to THC that, just might, create a false positive result on drug screening equipment (as metabolite or potentially interfering substances):



/-11-Nor-delta-9 -THC-COOH




The new approved drug screening equipment needs to be independently studied, by scientists in private practice, to determine, whether or not such metabolites or interferent substances may create false positives on the approved drug screening equipment,

We are particularly concerned about false positives during cold weather use in Canada.

The bottom line is that it may not be safe to drive ANY LENGTH OF TIME after consuming cannabis. That's what the Justice Committee of Parliament was told by the Chiefs of Police in September 2017. 28 days is about the shortest length of time that I have seen calculated, but some individuals will have a THC metabolite in their system for much longer than that.

There are some sites on the Internet that may help you find out more about how long you have to wait before you will probably pass a urine test. I don't endorse any of them but here are some  possible sources of further information on blood drug concentration calculation. Search for "blood drug concentration calculator Canada".

If you are charged with one of these offences your defence lawyer will need to be creative and find something wrong with the police procedure. Don't Give Up! Get good legal advice and contest the charge.