Abstract Defence lawyers sometimes argue that the presence of cannabinoid metabolites in the defendant's blood or urine resulted from passive unintentional inhalation of environmental cannabis smoke. It would be useful to be able to differentiate passive inhalation from active use so as to discourage the potential abuse of this phenomenon by the defence. Four cases from two jurisdictions in which passive cannabis smoking was used as a defence are presented to illustrate this dilemma. It remains impossible to define objectively an upper limit for blood and urine levels in cases of passive inhalation of cannabis from the environment. However, a claim of passive inhalation, or indeed ‘deliberate passive exposure’ could be discouraged by making it an offence to place oneself in a position of being ‘concerned’ in the use of the drug. The onus should be on the defendant to prove that he had not attempted to extricate himself from the situation, being aware of the smoking of cannabis in his immediate vicinity; ignorance would not be an excuse.