Edit: See also DPP v Kao  VSCA 273, where the Court of Appeal express their doubts [at 42] as to whether the mitigating principles in Verdins‘ case have any application to a psychosis resulting from the self-administration of drugs by an experienced user.
The principles of Verdins do not dictate the automatic mitigation of sentence in an offender simply because he or she had suffered or is suffering from an mental illness, however severe. Rather, Verdins requires scrutiny and assessment, based on cogent evidence, of the relationship between the mental disorder and the offending and other relevant matters.
In Edwards v The Queen  VSCA 87 the Court didn’t spend very long on the issue before approving Cannon J’s decision to treat use of amphetamine as an aggravating factor [at 23]. This appeal probably demonstrates a discrepancy between the lower courts and appellate authority on the use of intoxication as mitigation.
Section 20 of the Crimes (Mental Impariment and Unfitness to be Tried) Act 1997 provides for the statutory defence of mental impairment.
In R v Martin (No 1) (2005) 159 A Crim R 314, the accused adduced evidence that he suffered from cannabis-induced psychosis. A psychiatrist testified that the affliction caused aural or visual hallucinations, delusional beliefs and possibly impulsive reactions. The psychiatrist thought the accused suffered from this psychosis.
Bongiorno J held that mental impairment was synonymous with the previous common law concept of disease of the mind first articulated in McNaghten’s Case (1843) 10 Cl & Fin 200; 8 ER 718.
He referred to R v Radford (1985) 42 SASR 266; (1985) 20 A Crim R 388, approved in R v Falconer (1990) 171 CLR 30 at 53:
…I do not think that a temporary disorder or disturbance of an otherwise healthy mind caused by external factors can properly be regarded as disease of the mind as that expression is used in the M’Naghten Rules.
What that means is that transient or temporary altered states of consciousness — like drunkenness and drug-intoxication — are not ‘diseases of the mind’ or ‘mental impairment’.
In Martin (No 1), the psychiatrist conceded in cross-examination if the accused stopped taking cannabis, the psychosis stopped. For example, when the psychiatrist assessed Mr Martin, he was not psychotic.
On the evidence before him, Bongiorno J ruled the accused was not entitled to raise the defence of mental impairment as the cannabis-psychosis was temporary or caused by external factors.
As always, it’s important to consider the evidence in each case, but this decision might be useful for assessing some claims of mental impairment.