性能（英语：efficacy），又称为内在活性（英语：Intrinsic activity，缩写IA）在药理学中指药物在受体上能产生的最大反应。这和药物与受体的结合亲和性不同，也和测量效价强度的EC₅₀不同。

1956年，斯蒂芬森（Stephenson）创造出“性能”这个术语， 用来形容各种激动剂在结合相同数量受体时所引起的反应差异。高性能的激动剂与少量受体结合就能产生强烈反应；而低性能的激动剂与受体结合后较难稳定其激活的状态，所以即便全部结合，产生的反应也较弱。当没有受体储备时，低性能的激动剂不能引起最强烈的反应，有时也把它们称为部分激动剂。

Intrinsic activity (IA) or efficacy refers to the relative ability of a drug-receptor complex to produce a maximum functional response. This must be distinguished from the affinity, which is a measure of the ability of the drug to bind to its molecular target, and the EC₅₀, which is a measure of the potency of the drug and which is proportional to both efficacy and affinity. This use of the word "efficacy" was introduced by Stephenson (1956) to describe the way in which agonists vary in the response they produce, even when they occupy the same number of receptors. High efficacy agonists can produce the maximal response of the receptor system while occupying a relatively low proportion of the receptors in that system.