It has been known for more than 50 years that drugs are not chemically stable compounds providing the desired therapeutic effect and then being excreted from the body. Studies of drug metabolism can predict the consequences of interactions of drug to drug, food to drug, and herbal medicine to drug, and explain the unexpected patients' responses to the drug regimen, especially in long-term therapy and treatment with multiple drugs simultaneously. Drug metabolism includes absorption, distribution, biotransformation, and elimination reactions under the name ADME (Administration, Distribution, Metabolism, Excretion). Drug structure changes during the biotransformation reactions, thereby changing its physicochemical and biological properties. The role of biotransformation is detoxification and faster elimination of the drug, but in some cases toxicity is increased due to formation of toxic intermediates and metabolites. Besides inactive metabolites, biotransformation reactions of analgesics, substances used in therapeutic doses to relieve or eliminate pain, produce also active metabolites, and in some cases toxic metabolites responsible for adverse drug effects. Mechanisms of some biotransformation reactions of analgesics are not yet fully understood and the presence of certain metabolites in the bile, feces or urine is based on assumptions, so the metabolisms of some analgesics are still investigated.