controlled release
Controlled release is the most demanding member of the family: not just slow, but precisely metered. Think of an intravenous drip set to deliver an exact number of drops per minute — the goal is a predetermined, often constant release rate that holds the blood level inside a narrow target window.
Strictly, a controlled-release system aims for zero-order release, where the same amount of drug leaves the dosage form each unit of time regardless of how much is left. Osmotic pumps and well-designed reservoir devices approach this ideal, because their driving force (osmotic pressure or a saturated reservoir) stays nearly constant until the drug is almost gone.
In everyday and even regulatory language, controlled release is frequently used loosely as a synonym for sustained or extended release. The careful distinction is that controlled release promises rate control toward a defined profile, whereas sustained release only promises that release is prolonged.
An osmotic-pump nifedipine tablet (the OROS design) delivers the drug at a nearly constant rate over 24 hours, giving smooth blood-pressure control without sharp peaks.
An osmotic device approaching ideal zero-order delivery.
True zero-order controlled release is hard to achieve; many products labelled controlled release in fact follow first-order or Higuchi square-root-of-time kinetics.