Poison is a substance that harms or kills living things when it gets inside their bodies. A poison might cause illness, injury, or death depending on how much enters the body and how dangerous it is. Household cleaners, certain plants like nightshade, and some chemicals are poisonous if swallowed, breathed in, or absorbed through the skin.

Throughout history, people have understood that some substances are helpful in tiny amounts but deadly in larger doses. Ancient healers knew that certain mushrooms could cure diseases or cause death depending on the dose. This is why medicines come with careful instructions: the right amount helps you, but too much becomes poisonous.

The word also works as a verb. Someone might poison a water supply or poison food, which is a serious crime. You might also hear people say that mean gossip poisons a friendship, meaning it damages or destroys something healthy. Similarly, if bitter feelings poison someone's attitude, those negative emotions can corrupt their whole outlook.

The opposite of poison is an antidote, a substance that counteracts poison's effects. If someone swallows poison, doctors might give them an antidote to save their life. This is why poison control centers exist: to help people quickly when poisoning accidents happen.