… is from page 16 of Jeffrey Miron’s excellent 2004 monograph, Drug War Crimes:
Under legalization, the incidence of accidental poisonings or overdoses would not be zero, just as it is not zero for currently legal goods such as alcohol. But the rate of such incidents would decline significantly, since consumers would know the potency of the drugs they consume and have far greater confidence that the drugs contained the desired ingredients rather than unknown contaminants.
Because suppliers under prohibition need to hide their activities from the authorities, traffickers have an incentive to produce and ship drugs in the most concentrated and hence most easily concealed form (e.g., crack versus cocaine, heroin versus opium, spirits versus beer). This implies that prohibition expands the availability of potent forms of drugs or even helps create those more potent forms.