Common DUI Defences

NOTE: Criminal Code amendments establishing new or modified impaired driving offences came into force on December 18, 2018. Some of the information below may not apply to these. However, I have not yet removed that information from this article as charges for offences from before December 18, 2018, are still before the courts. For details on the amendments see New Laws on Impaired, Other Driving Crimes and for the corresponding penalties Penalties for New Impaired Driving, Exceed and Other Driving Crimes.


Numerous defences can be raised on a charge of Over 80.

Occasionally, a defence will be apparent from your own description of the investigation and the events leading up to your arrest. As evidence to support possible defences may vanish over time, it’s vital to see a lawyer as soon as possible. In some cases, a defence will emerge from a review of the police investigation materials such as note-book notes and videos.

But in many cases there is no apparent defence. Surprisingly, however (as can be seen from examples cited under Cases), even in such seemingly hopeless cases, a trial can succeed.

A few of the most common defences are described below and on the pages that follow. Availability of any defence depends on the facts of your case.

Breath readings unreliable

Where the first breath test is taken within two hours of the time of the alleged driving or care or control, you can win an acquittal by raising a reasonable doubt about the reliability of the breath test results. Only evidence that the breath instrument was malfunctioning or operated improperly is capable of raising such a doubt. Even if you produce such evidence (for example, by calling an expert on breath instruments), the Crown could still secure a conviction by showing that despite the defect, your blood alcohol level would still have been above the legal limit.

First test outside 2 hours
If the first breath test was taken more than two hours after the time of the alleged driving or care or control, you may be able to raise a reasonable doubt about the reliability of the breath test results by showing that your blood alcohol level at that time was within the legal limit. To mount such a defence, you will need a toxicologist (an expert on the body’s absorption and elimination of alcohol) or a report from one. The toxicologist can calculate your BAC based on your weight and the amount and time of alcohol consumption.

Unless the judge finds the evidence about how much you drank unreliable, your testimony will throw doubt on the readings obtained by police. In assessing your evidence about alcohol consumption, the judge cannot take into account your breath test results, however high. This follows from a December 2005 decision of the Supreme Court of Canada (see paragraph 43).

Bolus Drinking

In cases where the first breath test was taken over two hours after the alleged offence, you may have a “bolus drinking” defence. If there is evidence suggesting consumption of alcohol just before police pulled you over (e.g., half empty liquor bottle within arm’s reach), the Crown may be unable to refute the possibility that the alcohol had not yet entered your blood by the time of the offence. It can take about 20 minutes for it to do so.

Were you over the limit?
Were you over the legal limit at the time of the alleged offence? Use the Calculator to approximate your blood alcohol level.

On an Over 80 charge, the Crown must prove your BAC was over the legal limit at the time of driving. But because police do the breath instrument test only afterward, the Criminal Code presumes the result – the lower of the two readings obtained from the breath samples – reflects your BAC at the time you are stopped.

The presumption applies only if the tests are started within two hours. If the first test is carried out more than two hours after the time of driving (or care or control), the Crown will need a toxicologist to calculate your BAC at that time. If the Crown fails to produce evidence from a toxicologist at your trial, the Over 80 charge cannot be proven.

Last drink defence
If the breath instrument readings are slightly over 80 and you guzzled a drink just before being stopped, you may have a “last drink” defence. As it takes time for alcohol to enter the bloodstream, a toxicologist may be able to testify you were under 80 at the time of the stop because the alcohol had not yet been fully absorbed. The last drink defence is very effective because you do not have to question the reliability of the breath instrument results. However, your evidence must also show that the breath instrument results accurately reflect your BAC at the time of the tests.