sustained release
Picture a candle versus a flare: the flare burns out in seconds, the candle keeps a steady, gentle flame for hours. A sustained-release formulation is the candle — it lets the drug out slowly and gradually so a therapeutic level is held over an extended period instead of spiking and fading.
Sustained release usually combines an initial dose to reach the effective level quickly with a maintenance portion that trickles out afterward. The slowdown is achieved by physical barriers — a polymer matrix the drug must diffuse through, a coating it must permeate, or a material that erodes over time. The aim is fewer doses per day and steadier exposure.
In casual use sustained release and extended release are often treated as synonyms, and many regulators group both under modified release. Strictly, sustained release emphasises maintaining a level over time, while it does not promise the constant, predetermined rate implied by true controlled release.
A once-daily sustained-release theophylline tablet keeps airway-relaxing levels steady overnight, avoiding the dips that a plain tablet taken at bedtime would cause.
Steadier overnight levels from a single dose.
Many sustained-release products release drug by first-order kinetics, so the rate tapers as the dose is depleted rather than staying truly constant.