Pharmacologically active substances comprise drugs and poisons. Drugs possess therapeutically beneficial, and poisons possess deleterious effects. Drugs should be named by their INNs and not by their brand names. There are substantial cultural differences in the use of a given drug in various countries. Drugs should be classified according to their mechanism of action, and traditional slang terms should be avoided. Medicines are pharmaceutical preparations of drugs for use in humans. Development of a drug into a medicine comprises preclinical and clinical development. The latter is divided into three phases. Pharmacodynamics analyzes the effects of pharmacologically active substances on the human organism. Receptors, enzymes, ion channels, and transporters are the most important target classes for drugs. Receptors are divided into GPCRs, ligand-gated ion channels, TK-linked receptors, and NRs. Receptors are activated by agonists. Antagonists block the effects of agonists. The function of enzymes and transporters is reduced by inhibitors. Ion channel function is reduced by blockers and increased by activators. Complete concentration-response relationships are required to assess the effects of drugs with the parameters EC50, IC50, and intrinsic activity. The therapeutic index is a measure for the safety of a drug. Many drugs have a small therapeutic index and should, therefore, be dosed prudently. Some drugs with a small therapeutic index are even available OTC.