RE:RE:RE:Canada is making policyapologies for the last post. scroll down to see the law dealing with the matter.
(3.6) If the analysis of a sample provided under paragraph (3.4)(b) 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 motor vehicle, vessel, aircraft or railway equipment, 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 motor vehicle, vessel, aircraft or railway equipment and, on proof of the person’s impairment, to have been the cause of that impairment.