 Solution

On the stipulated figures, a 20% decline from 200 cocaine-related deaths means about 40 fewer deaths, against about 10 attributed to mephedrone:That bookkeeping is relevant to a population-harm assessment only if the avoided deaths really result from substitution, the new deaths are counted comparably, and overlapping drug attributions are not counted twice. A change in the legality of one drug is an intervention affecting behaviour; the before-and-after association alone does not identify its causal effect.

Distinguish per-user danger from total population harm. More use can raise total deaths even for a less dangerous drug, and an increase can involve new users who would otherwise use no drug. Conversely, restricting a substitute may move users back to more dangerous alternatives. The relative fatality risks do not mechanically determine legal classification: nonfatal harm, dependence, dosage and purity, co-use, the composition of users, enforcement and supply effects, and uncertainty in the estimates all matter. Any policy assessment should compare plausible behavioural responses to the alternatives, including displacement of both cocaine and MDMA, rather than extrapolate from ten deaths alone.