In the case of tobacco, Manning estimates the gross cost of medical care for smoking-related diseases at $.26 a pack, or just over one penny per cigarette. This turns out to be largely compensated by savings in retirement pensions and nursing home care for smokers. The final balance is highly sensitive to technical assumptions about the economic discount rate, and can even be made to show net external benefits at interest rates under 3%. Manning's final net estimate of $.15 per pack assumes a 5% interest rate.
By estimating the equivalency between joints and cigarettes, one can translate these costs to marijuana. On a weight-for-weight basis, pot smokers inhale about four times as much noxious tars as cigarette smokers;14 as we have seen, however, the average joint weighs about half as much as a cigarette. Also, cannabis lacks nicotine, a leading factor in tobacco-related heart disease. It seems reasonable on this basis to suppose that a joint is equal to less than two cigarettes, putting the net external cost of marijuana smoking at under 1.5 cents per joint.
One fault in Manning's accounting of external costs is that it excludes the costs of second-hand smoking, which he estimates at $.23 per pack, on the questionable grounds that these costs are mainly internal to the users' families. We treat them here as external costs instead. There are grounds to think that passive smoking is of much less concern with cannabis since pot smokers emit less smoke than cigarette smokers. It therefore seems reasonable to conclude that the total smoking-related costs of active and passive pot smoking are unlikely to exceed two cents per joint.
Turning to alcohol, Manning concludes that the net medical-less- pension costs of alcoholism-related disease are $.26 for every "excess ounce" of alcohol, which is defined to mean an ounce in excess of one per day (Manning does not try to account for the possibility that moderate consumption may actually extend life). These costs turn out to be greatly outweighed by the cost of alcohol-related accidents, which he estimates at $.93 per excess ounce. This figure includes traffic accidents to third parties caused by drunken drivers, but does not appear to include other alcohol-related accidents. Also missing from Manning's account are the external costs of alcohol-related violence. Altogether, Manning concludes that the total cost of alcohol is $1.19 per excess ounce, or $.48 per ounce when averaged over all alcohol drunk.
While the cost of alcohol seems clearly dominated by accidents, it is unclear how to relate these to marijuana. The burden of expert opinion appears to be that marijuana is less of an accident risk than alcohol, though this is disputed.15 Studies of fatal car accidents indicate that, at least on the road, marijuana tends to be a secondary risk factor compared to alcohol.16 On the other hand, one survey of trauma patients found that with respect to all accidental injuries, cannabis may be every bit as much a risk factor as alcohol.17 In terms of intoxicating potential, one joint probably lies between one ounce and one excess ounce of alcohol. At the high end, if one equates a joint with one excess ounce, the accident costs of pot would be $.93 per joint. More reasonably, one could equate a joint with an "average" ounce of alcohol, the accident costs of which work out to $.38. There are reasons to favor a lower external cost on marijuana relative to alcohol, notably the fact that marijuana tends to suppress violence, whereas alcohol tends to aggravate it. From this perspective alone, an overall shift from alcohol to marijuana may be desirable.