鸦片类药物（Opioid）是一类具有吗啡作用的化学物质。它的主要用途是镇痛。鸦片类药物通过存在于中枢神经系统和消化系统的鸦片类受体（Opioid receptor）起作用。这些鸦片类受体能引发有益的药物作用或药物不良反应。

鸦片类药物可划分为以下几类：

虽然鸦片剂（Opiate）经常被作为是鸦片类药物的同义词，但更确切地说鸦片剂一词应只限于天然鸦片生物碱和它们的半合成衍生物。一些具有鸦片类药物效力的次要的鸦片生物碱和其他一些物质也在自然界中被发现，其中包括紫堇属和鼠尾草属植物中的生物碱。在所有超过120个罂粟物种当中，只有两个可以制造吗啡。经发现，人体和一些动物的身体会天然产生少剂量的吗啡和可待因。

Opioids are substances that act on opioid receptors to produce morphine-like effects. Opioids are most often used medically to relieve pain. Opioids include opiates, an older term that refers to such drugs derived from opium, including morphine itself. Other opioids are semi-synthetic and synthetic drugs such as hydrocodone, oxycodone and fentanyl; antagonist drugs such as naloxone and endogenous peptides such as the endorphins. The terms opiate and narcotic are sometimes encountered as synonyms for opioid. Opiate is properly limited to the natural alkaloids found in the resin of the opium poppy although some include semi-synthetic derivatives. Narcotic, derived from words meaning numbness or sleep, as an American legal term, refers to cocaine and opioids, and their source materials; it is also loosely applied to any illegal or controlled psychoactive drug. In other jurisdictions all controlled drugs are legally classified as narcotics. The term can have pejorative connotations and its use is generally discouraged where that is the case.