Intoxication is not a defence in itself but rather it can have a significant bearing on a jury's assessment of whether the required element of a crime existed. The jury must be satisfied that the accused's actions were voluntary or that the accused had the relevant intention to commit the offence. As such, it might be something relied upon by the defence in order to cast doubt upon the criminal prosecution and the evidence of intention or voluntariness. Intoxication may be by way of drugs, alcohol or a combination of both.
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 15 it is stated that it was in his Honour's opinion incumbent on a judge to carefully instruct the jury, in a manner long recognised 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.
Gibbs J pointed out in the case of Viro v R (1978) 141 CLR 88 that the jury should also be told that the fact that the accused was intoxicated, whether by drink or by 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. This form of direction can be referred to as the 'negative direction' in that 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 prosecution 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.