This chapter discusses about the drug use in special population/diseases/physiological conditions. Certain pathological and physiological condition can change pharmacokinetics of a drug affecting its action. Hence, dose adjustment is needed in these conditions. Very old and very young patients are more prone to adverse drug reactions (ADRs). When the drug manufacturer does not provide adequate information about pediatric dosage, there can be substantial risk in deriving a dose for children and infants from an adult dose. Drugs given during pregnancy may affect fetus. Thalidomide is a classical example of teratogenic effects. There are so many physiological changes during pregnancy which can affect drug action. In pregnant state pharmacokinetics and pharmacodynamic of a drug change considerably. Most drugs are secreted in breast milk in very small quantities. These do not affect suckling infant. However there are few drugs, which readily enter breast milk and adversely affect nursing infant. Care should be taken while prescribing drugs to lactating mothers.