The functional relationship between the dosage of a drug and the size or strength of a physiological or behavioural response to it. It is almost invariably monotonically increasing, with larger dosages of the drug producing larger responses, at least within a certain range, but very low and/or very high dosages may not follow this pattern. For example, a low dosage of adrenalin (epinephrine) or acetylcholine produces a slight drop in blood pressure, whereas a high dosage produces a large increase in blood pressure, and a low dosage of caffeine may enhance typing skill whereas a larger dosage impairs it. Also called the dose-response relation(ship). See also initial values law.