Purity is a measure of how much of some substance is “what it says on the tin”. Potency in contrast refers to the dose necessary for a substance to exert its effect on the body. Ivory Soap, famously, is 99.4% pure. But its potency when ingested is far less than that of 1% pure heroin.
This ends up mattering more than one might think in drug policy debates. Policymakers and parents get scared when they hear that an addictive drug is pure. But a pure drug can be safer than an impure drug if the potency of the latter exceeds the potency of the former. Even within the confines of the same potent drug, purity can increase safety if the impurities are nasty things such as levamisole or strychnine.One concrete area where this misunderstanding is influential and sometimes exploited for effect concerns methamphetamine. A prescription requirement for the pseudoephedrine-containing cold medicines that are used to make meth would virtually wipe out domestic meth labs. Opponents of this policy sometimes try to scare people by saying that the result would be an influx of “much purer” methamphetamine made without pseudoephedrine (PSE) in Mexican superlabs (PSE imports are banned in Mexico). Yes, that form of methamphetamine is purer, but it’s less potent and hence less dangerous than methamphetamine made from PSE, even before you count up the burn injuries, explosions and fires that small-scale domestic meth labs cause.