oral suspension
An oral suspension is a liquid medicine in which tiny solid drug particles float dispersed through the liquid rather than being dissolved — like cocoa powder stirred into milk that you must shake before drinking. The fine print on the bottle, “shake well before use,” is the whole story.
It is the go-to oral liquid for drugs that simply will not dissolve well in water. The solid particles are wetted and suspended in an aqueous vehicle thickened with a suspending agent that slows their settling, while sweeteners, flavours and a preservative make it palatable and microbially stable. A well-designed suspension settles slowly and, even when it does settle, redisperses into a uniform liquid with a quick shake.
The central challenge is keeping the dose uniform. Because the drug is undissolved, every bottle is a balancing act between particles sinking under gravity (described by Stokes' law) and the formulator's tricks to slow or reverse that. Controlled flocculation — letting particles form loose, easily redispersed clumps — is often preferred over a hard, caked sediment that will not shake back up.
Amoxicillin is dispensed as a powder for oral suspension that the pharmacist reconstitutes with water, because the drug is unstable in water for long and must be used within a couple of weeks once mixed.
Undissolved drug particles must be shaken back into uniform suspension before each dose.
A deflocculated suspension looks elegant and stays clear longer, but if it does settle it can cake hard; a controlled-flocculated one settles faster yet redisperses easily — formulators usually favour the latter for dose uniformity.