In the context of biology, poisons are substances that can cause damage, illness, or death to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism.

Paracelsus, the father of toxicology, once wrote: "Everything is poison, there is poison in everything. Only the dose makes a thing not a poison."

In medicine (particularly veterinary) and in zoology, a poison is often distinguished from a toxin and a venom.

Toxins are poisons produced via some biological function in nature, and venoms are usually defined as biologic toxins that are injected by a bite or sting to cause their effect, while other poisons are generally defined as substances which are absorbed through epithelial linings such as the skin or gut.

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