毒物是对生物造成不适反应的物质的总称。

毒物对生物体造成的影响因种类不同各异，不适反应的类型以及程度也各不相同。另外对于有的生物来说具有毒性而对于别的生物来说无毒的“选择毒性”在自然界中也存在。比如，抗生素对某些微生物具有毒性，但对于其他生物基本无害。此外生物所必需的各种微量化合物，如维生素，矿物质等超过一定量后也会出现毒性。例如钙是骨骼形成所必需的，但是摄取过多钙会损伤肾脏。

日常生活中称为「毒物」的除了急性或者慢性毒性的物质以外、还有致癌或者导致畸变的物质，极端的例子有如沙利度胺是一种强力的致畸性物质但是其毒性极弱。

In biology, poisons are substances that cause disturbances in organisms, usually by chemical reaction or other activity on the molecular scale, when an organism absorbs a sufficient quantity.

The fields of medicine (particularly veterinary) and zoology often distinguish a poison from a toxin, and from a venom. Toxins are poisons produced by organisms in nature, and venoms are toxins injected by a bite or sting (this is exclusive to animals). The difference between venom and other poisons is the delivery method. Industry, agriculture, and other sectors use poisons for reasons other than their toxicity. Pesticides are one group of substances whose toxicity is their prime purpose.