Science Cannabis

202 THE SCIENCE OF MARIJUANAFigure 5.1. Effects on survival of stopping smoking tobacco before the ageof 35. Results from a survey of about 40,000 British doctors; those whocontinued smoking had their life expectancy reduced by about 8 years, butthose who gave up smoking before age 35 had a life expectancy not significantlydifferent from that of nonsmokers. From Doll et al. (1994). Reprintedwith permission from BMJ Publishing Group.15-20 cigarettes commonly consumed by tobacco users —each joint isliable to deposit 4-5 times more tar in the lungs than a tobacco cigarette.The tar exposure is thus similar, except for the large numbers who smokemarijuana with tobacco; for them the hazards are compounded.At the moment the jury is out on the link between marijuana andcancer, although there has been some concern about reports of an increasednumber of cancers of the tongue and larynx in young peoplewith a history of heavy marijuana use (see Tashkin, 1999). These arebased, however, on very small numbers and no cause and effect relation-