Mortality

Mortality refers to the rate of death within a defined population over a stated period — not any single death, but a ratio comparing the number of deaths against the number of people exposed or at risk. The figure appears in harm-reduction and epidemiological literature because it provides one of the clearest available comparisons of how lethal different substances are, and where intervention carries the most weight.

The figure is meaningful only alongside its denominator: deaths per user, per episode of use, per year of exposure, or per defined population. A substance responsible for many deaths in absolute numbers may carry lower mortality than a rarer one if its user base is large enough.

How it is done

Mortality data is assembled from overlapping sources: death certificates, medical examiner and coroner records, hospital discharge data, and linked population registries. Each assigns cause of death through established classification codes, and many datasets distinguish direct mortality — where a substance caused the physiological event leading to death — from indirect mortality, where it contributed to an accident, suicide, or progression of untreated illness.

Researchers also separate acute mortality, covering events such as overdose or acute toxicity, from chronic mortality covering organ failure or disease accumulated over long-term use. Comparing figures across studies requires confirming that they share the same denominator and the same attribution method. A rate expressed per hundred thousand general-population members counts differently from one expressed per thousand people who actively use a given substance.

What it cannot tell you

A mortality figure does not establish what caused the death. It records that death occurred and, where attribution is possible, associates it with a substance — but attribution depends entirely on how deaths were classified, and classification varies between jurisdictions, time periods, and coding practices. Two credible sources can report very different rates for the same substance without either being factually wrong.

The most common wrong conclusion drawn from a low mortality figure is that the substance is safe, or safer than its reputation suggests. A low rate may reflect rare use, a young user population not yet old enough to show chronic harms, or a gap in toxicology screening — not an inherently low hazard. A high rate can similarly mislead: it may reflect that users of that substance routinely combine it with others, and the combination, not the substance alone, is what drives the outcome.

Mortality data describes patterns at the population level. It carries no information about individual risk in a specific situation, for a person with specific health conditions, or at a specific level of use. The absence of recorded deaths in a dataset is not evidence that deaths have not occurred.