Lethal dose

The lethal dose of a substance is the amount capable of causing death. In research, this is most often expressed as the LD₅₀ — the median lethal dose — the quantity at which half of a population of test animals dies under controlled conditions. The figure is reported in milligrams of substance per kilogram of body weight (mg/kg) and varies by species, route of administration, and study design.

An absent LD₅₀ value does not mean a substance is safe at any dose; it means no controlled study has measured one. For most compounds in this encyclopedia — particularly plant medicines, novel psychoactives, and substances where lethal exposure in humans would be unethical to study — no formal figure exists in any species.

How it works · its role

LD₅₀ figures come from acute toxicity studies in animals, most commonly rats or mice. A range of doses is administered to groups, and the dose at which roughly half die within a fixed observation window is interpolated from the results. The number is a statistical estimate of a population midpoint, not a precise threshold.

Two factors limit how these figures translate to human risk. Drug metabolism differs substantially across species: a dose lethal to a rat may sit at a very different ratio to a human equivalent, and that relationship is not always predictable in either direction. Additionally, the LD₅₀ captures only acute toxicity; it says nothing about cumulative damage, neurological harm, or the long-term effects of repeated sub-lethal exposure.

A closely related concept is the therapeutic index: the ratio of the lethal dose to the dose that produces a desired effect. A narrow therapeutic index means effective and dangerous doses sit close together, demanding more precise dosing and leaving little margin for error.

Relevance to substances & effects

Classic serotonergic psychedelics such as psilocin and LSD have exceptionally high LD₅₀ values in animal models, meaning the acute lethal dose in test species lies far above the active dose. This is one reason fatalities attributed solely to these compounds are exceedingly rare, though it does not make them risk-free at high doses or in combination.

Opioids sit at the opposite end of the spectrum. Their therapeutic index is narrow — the gap between a dose that produces pain suppression and one that suppresses breathing fatally is small — and that gap narrows further with polydrug, individual variation in metabolism, or exposure to fentanyl analogues of uncertain potency.

Route of administration always matters when interpreting a lethal dose figure. The oral LD₅₀ of a substance can differ dramatically from its intravenous equivalent, because absorption rate and peak plasma concentration change the risk profile. Where a lethal dose figure appears in this encyclopedia, it will carry its species and route of administration; a bare number without those qualifiers carries no usable meaning.