Tolerance

Tolerance is the body's adaptation to a drug with repeated use, so that the same dose produces a smaller effect over time and larger doses are needed to reach the original result. It develops through several mechanisms — the brain adjusts receptor numbers and sensitivity, and the body may metabolize the drug faster. Tolerance develops at different rates for different effects of the same drug, which is itself a hazard: for opioids, tolerance to the euphoric effect often builds faster than tolerance to the life-threatening respiratory depression, narrowing the margin of safety. Related concepts include cross-tolerance (tolerance to one drug reducing the effect of a similar one) and reverse tolerance/sensitization (rarely, becoming more sensitive with use).

The single most dangerous fact about tolerance is that it fades. After a period of reduced use or abstinence — leaving detox or rehab, release from incarceration, illness, or simply a break — tolerance drops, often faster than people expect. Returning to the dose that felt normal before can then cause a fatal overdose. This 'lost tolerance' effect is a leading cause of overdose death, especially with opioids: the post-release and post-detox periods carry sharply elevated overdose risk for exactly this reason. Tolerance also drives escalation and dependence — needing ever-higher doses to feel an effect or to feel normal pulls users toward heavier, costlier, riskier use, and toward withdrawal when they stop. Cross-tolerance means someone tolerant to one depressant (say alcohol) may underestimate a different one (a benzodiazepine), while dangerous additive effects on breathing still stack — tolerance to the 'high' does not protect against the combined respiratory risk. Because tolerance to different effects builds unevenly, a dose that feels 'not strong enough' can still be physically dangerous. The practical harm-reduction takeaways: after any break in use, a previously normal amount may be far too much; never assume past tolerance still applies. If someone who has been abstinent relapses and becomes unresponsive or has slowed breathing, treat it as an overdose emergency — call 911, and give naloxone if opioids may be involved. This page has not yet been medically reviewed.