cannabis smoking may have a greater potential than tobacco smoking to cause lung cancer [1–4]. Cannabis smoke is qualitatively similar to tobacco smoke, although it contains up to twice the concentration of the carcinogenic polyaromatic hydrocarbons [1]. Cannabis cigarettes are less densely packed than tobacco cigarettes, and tend to be smoked without filters [2] to a smaller butt size [3], leading to higher concentrations of smoke inhaled. Further- more, smokers of cannabis inhale more deeply and hold their breath for longer [4], facilitating the deposition of the carcinogenic products in the lower respiratory tract. These factors are likely to be responsible for the five-fold greater absorption of carbon monoxide from a cannabis joint, compared with a tobacco cigarette of similar size despite similar carbon monoxide concentrations in the smoke inhaled [4]. Several studies have demonstrated pre-cancerous histological [5, 6] and molecular [7] abnormalities in the respiratory
tracts of cannabis smokers, and the carcinogenic effects of cannabis smoke have been demonstrated
in vitro [8] and in different in vivo animal models[1, 9, 10]. Conversely, there is also evidence that
delta-9-tetrahydrocannabinol may have anti-carci-nogenic effects [11–13].