The skull and crossbones symbol is used to label something with poison in it

Poisons are substances which cause death or injury when taken in by a living thing. It may be taken in as drink or food, or absorbed through the skin. The damage is usually done by a chemical reaction. The effect of the poison varies with the amount which is absorbed (taken in or inhaled). Substances which are poisonous are called toxic, but any substance can be toxic if too much of it is consumed. If poisoning causes death, it is lethal poison.