firstdose

First dose refers to the initial amount of a drug given to a patient at the start of therapy. It is intended to initiate therapeutic effects while managing the risk of adverse reactions. The first dose is chosen based on the drug’s pharmacokinetics (absorption, distribution, metabolism, elimination) and pharmacodynamics, as well as patient factors such as age, weight, organ function, coexisting conditions, and potential drug interactions. In some regimens, a loading dose may be used to rapidly achieve target blood concentrations, while other therapies employ gradual titration to minimize adverse effects.

First-dose phenomenon describes certain drugs that produce immediate or exaggerated responses after the initial administration, such as orthostatic hypotension with some antihypertensives or dizziness with alpha blockers. This has implications for monitoring, dosing in special populations, and patient education.

Dosing strategies often include hospital observation after the first dose for high-risk medicines, or patient-specific instructions for outpatient settings. Safety considerations emphasize monitoring vital signs, adverse effects, and laboratory parameters.

Examples of drugs and contexts where the first dose matters include antibiotics started empirically, antidepressants during initiation, anticoagulants, and antihypertensives. Special populations such as the elderly or those with renal or hepatic impairment may require dose adjustments and slower titration.

See also: loading dose, maintenance dose, titration, half-life, peak concentration (Cmax), trough level.