Final answer:
The time between the administration of a drug and the first appearance of its effects is called onset time. Onset time should not be confused with half-life, duration, or clearance time, which are all different pharmacokinetic measures.
Step-by-step explanation:
The time between the administration of a drug and the first appearance of its effects is called onset time. This term refers to how long it takes for a drug to start having its intended effect after it's been administered. A drug's onset time can vary based on the method of administration; for example, intravenously administered drugs often have a quicker onset time compared to drugs that are given orally or intramuscularly.
It's important not to confuse onset time with other pharmacokinetic terms such as half-life, which is the period of time it takes for the concentration of a drug to reduce to half of its initial value in the body. This measurement indicates how long a drug stays active in the system. Duration refers to how long the effects of the drug last, while clearance time is about how long it takes for the drug to be eliminated from the body.