半数中毒量（median toxic dose，TD₅₀）为毒理学名词，是指一药物或是毒素造成50%比例中毒的剂量。不过若要数值有意义或是可以用在实际应用上，需说明其中毒的种类。半数中毒量的剂量一般会比半数有效量 （ED₅₀）要大，但是比半数致死量（LD₅₀）要小。不过有些高效价的毒素（像是及洛芬太尼及肉毒杆菌毒素等）其半数中毒量和半数有效量（ED₅₀）差异不大，一般会定义为相同。既然中毒不一定会致命，因此半数中毒量会远低于半数致死量，半数致死量也可以视为是半数中毒量的上限。若根据研究，某一毒素的结果不会致命，就会用半数中毒量定义其这类的毒性。毒性反应有不同的定义，而且有些药物在治疗时有毒性（如化疗），会造成半数中毒量和半数有效量（ED₅₀）的混淆。毒性反应的结果一般包括癌症、失明、贫血、生育缺陷等。

In toxicology, the median toxic dose (TD₅₀) of a drug or toxin is the dose at which toxicity 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, botulinum toxin) 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 (LD₅₀), and the latter can be considered an upper bound 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) which can lead to confusion and contention regarding a substance's TD₅₀. Examples of these toxic endpoints include cancer, blindness, anemia, birth defects, etc.