toxicology Toxicology is a scientific discipline, overlapping with biology, chemistry, pharmacology, and medicine, that involves the study of the adverse effects of chemical substances on living organisms and the practice of diagnosing and treating ex ...

, the median toxic dose (TD₅₀) of a drug or toxin is the dose at which

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacteria, bacterium, or plant, as well as the effect o ...

occurs in 50% of cases. The ''type'' of toxicity should be specified for this value to have meaning for practical purposes. The median toxic dose encompasses the category of toxicity that is greater than half maximum effective concentration (ED₅₀) but less than the median lethal dose (LD₅₀). However, for some highly potent toxins (ex.

lofentanil Lofentanil or lofentanyl is one of the most potent opioid analgesics known and is an analogue of fentanyl, which was developed in 1960. It is most similar to the highly potent opioid carfentanil (4-carbomethoxyfentanyl), only slightly more poten ...

botulinum toxin Botulinum toxin, or botulinum neurotoxin (commonly called botox), is a neurotoxic protein produced by the bacterium ''Clostridium botulinum'' and related species. It prevents the release of the neurotransmitter acetylcholine from axon en ...

) the difference between the ED₅₀ and TD₅₀ is so minute that the values assigned to them may be approximated to equal doses. Since toxicity need not be lethal, the TD₅₀ is generally lower than the

median lethal dose In toxicology, the median lethal dose, LD50 (abbreviation for " lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 is a toxic unit that measures the lethal dose of a given substance. The value of LD50 for a substance is the dose re ...

(LD₅₀), and the latter can be considered an

upper bound In mathematics, particularly in order theory, an upper bound or majorant of a subset of some preordered set is an element of that is every element of . Dually, a lower bound or minorant of is defined to be an element of that is less ...

for the former. However, since the toxicity is above the effective limit, the TD₅₀ is generally greater than the ED₅₀. If the result of a study is a toxic effect that does not result in death, it is classified as this form of toxicity. Toxic effects can be defined differently, sometimes considering the therapeutic effect of a substance to be toxic (such as with

chemotherapeutics Chemotherapy (often abbreviated chemo, sometimes CTX and CTx) is the type of cancer treatment that uses one or more anti-cancer drugs ( chemotherapeutic agents or alkylating agents) in a standard regimen. Chemotherapy may be given with a cura ...

) which can lead to confusion and contention regarding a substance's TD₅₀. Examples of these toxic endpoints include cancer, blindness, anemia, and birth defects.

References

Toxicology {{med-toxic-stub