Patients were asked to complete a form asking about their recreational drug use in the last 6 months and also to specify frequency of illicit drug use. All patients completed a form (n = 57) of whom nine patients reported cannabis use in the last 6 months and eight were regular users. This is a prevalence of 14% in this day surgical population.
Government figures confirm that recreational cannabis use has increased significantly in the last 10 years  and therefore the likelihood of a patient presenting for anaesthesia who is a regular or occasional user has risen. As cannabis is very slowly eliminated, the tissue half-life is approximately 7 days , it may be present in the body for many weeks after abstinence and continue to react cumulatively with sedative agents .
Cannabis smoking is also associated with upper airway irritability as well as the impairment in lung function that is seen in tobacco smokers. It has been estimated that 3–4 cannabis cigarettes smoked daily are equivalent to 20 tobacco cigarettes in terms of causing acute and chronic bronchitis and damage to the bronchial tissue . There have also been reports of acute upper airway oedema and obstruction in cannabis smokers undergoing general anaesthesia . Concern has been expressed in the literature about adverse psychiatric and autonomic reactions in patients who are regular cannabis users receiving anaesthetic agents .
In light of the increasing use of cannabis in the general population, we would recommend that questioning about illicit drug use becomes a part of routine preanaesthetic assessment. In addition, anaesthetists should have a high index of suspicion of illicit drug use in patients who prove unusually difficult to ‘settle’ with no other easily identifiable cause as unrecognised cannabis users can present particular problems in anaesthetic management.