‘How drugs work’ outlines the basic mechanisms of pharmacology. Drugs are chemicals that can be naturally occurring or man-made, and which can be administered in a variety of ways. They can act on receptors—often highly specific proteins in cells which can up-regulate or down-regulate processes—or on other targets, such as DNA or enzymes. The molecular action of drugs can be investigated in a lab, but the effects on the whole organism are more important. Effective doses need to be determined, taking into account metabolic rates, drug interactions, and side effects. Prolonged drug use can cause tolerance and substance addiction.