A substance which by chemical action and at low dosage can kill or injure living organisms. Broadly defined, poisons include chemicals toxic for any living form: microbes, plants, or animals. For example, antibiotics such as penicillin, although nontoxic for mammals, are poisons for bacteria. In common usage the word is limited to substances toxic for humans and mammals, particularly where toxicity is a substance's major property of medical interest. Because of their diversity in origin, chemistry, and toxic action, poisons defy any simple classification. Almost all chemicals with recognized physiological effects are toxic at sufficient dosage. The same compound may be considered a drug or a poison, depending on dosage, effect, or intended use.