Intoxication alone cannot provide a defence for an accused person. However, evidence of intoxication can have significant bearing on a Jury’s assessment of whether the required mental element of a crime existed.
The law regarding the relevance of evidence of intoxication was clearly stated by Winneke, P in the decision of R v McCullagh  VSCA 163 at paragraph 21:
In this case where, as I have said, the evidence relating to drug intoxication combined with fatigue was capable of raising a doubt in the minds of the jury as to the requisite intent, it was not sufficient for the judge to simply tell them that the Crown bore the onus of proving, to the appropriate standard, that the applicant had, at the relevant time, the requisite intent. He should further have told the jury, in relation to that issue, that the onus lies upon the Crown to remove any reasonable doubt from their minds which may have been raised by the evidence of the accused’s drug and fatigue. He should also have told them, by reference to the evidence, that the Crown must persuade them beyond reasonable doubt that the accused’s state of drug intoxication , combined with fatigue, was not such as to deny the relevant state of mind which might otherwise be found from the other evidence in the case. In a case such as this, it is only by giving specific directions of the kind to which I have referred that the judge can comply with the fundamental obligation of assisting the jury to apply the relevant law to the facts and issues raised by the evidence in the case.
A discussion of the issue is found elsewhere in the decision of Winneke, P (paragraph 15 onwards):
Having regard to the nature and content of the evidence before the jury, and its relevance to the only defence made by the applicant to the charge, it was, in my opinion, incumbent upon the judge to carefully instruct the jury, in a manner long recognized by the law, as to how they should use the evidence of drug intoxication and fatigue in resolving the issue of whether the Crown had established the requisite intent necessary to support the charge of murder. Thus, in a case such as this where there was evidence, fit to be considered by the jury, that the applicant was intoxicated as the result of drug ingestion combined with fatigue, it was not sufficient to simply tell the jury that the Crown must prove beyond reasonable doubt that the accused in fact formed the requisite special intent. As Gibbs, J. pointed out in Viro v. R.:
“They should also be told that the fact that the accused was intoxicated, whether by drink or drugs or a combination of both, may be regarded for the purpose of ascertaining whether the special intent in fact existed. It should be explained that evidence that the accused was intoxicated will not in itself entitle him to an acquittal … . However, the jury should be told that if, because of the evidence as to the effect of the intoxication or otherwise, they are not satisfied that the accused did in fact have the necessary intent, they must acquit of the crime which involves that intent.”
Such a direction is sometimes called, inelegantly, the “negative direction”. Rather it is a specific direction designed to ensure that the jury understands the relevance of the evidence of intoxication to the issue of intent and the need for the jury, having regard to that evidence, to be satisfied that the Crown had excluded the possibility that, because of the combination of drug intoxication and sleep-deprivation, the accused had not formed the requisite intent to kill or cause really serious injury. In R. v. Graham John Coleman, Hunt, J. described the necessary directions in the following way:
“The trial judge should direct the jury that the Crown must establish that the accused had in fact formed the state of mind which is relevant to the offence charged. In relation to that issue (if intoxication has been sufficiently raised in the evidence), the judge should tell the jury that the onus lies on the Crown to remove any reasonable doubt from their minds which may have been raised by the evidence of the accused’s intoxication . To do that the Crown must persuade them beyond reasonable doubt that the accused’s state of intoxication was not such as to deny the existence of the relevant state of mind which may otherwise be apparent from all the other evidence in the case.”
Hunt, J. went on to point out that, whilst these directions are necessary in cases where intoxication is raised on the issue of the formation of intent, the judge should balance them by telling the jury that intoxication does not amount, in itself, to a defence; and that in many cases it does no more than remove inhibitions or self-restraint and include a sense of self-confidence and, perhaps, aggression.