Clinical pharmacology encompasses all aspects of the relationship between drugs and humans. For healthcare professionals, clinical pharmacology is important because it is the scientific discipline that underpins the rational prescribing of medicines to alleviate symptoms, treat illness and prevent future disease. Medicines contain drugs (the specific chemical substances with pharmacological effects), either alone or in combination, in a formulation mixed with other ingredients. The beneficial effects of medicines must be weighed against potential adverse drug reactions and interactions, often caused by injudicious prescribing decisions and by prescribing errors, and their cost.