Suspensions for parenteral use are required when the drug is insoluble or poorly soluble in the aqueous vehicle. These are preferred for depot release of drug.

Some of the parameters to be considered while formulating suspensions are wettability, rate of sedimentation, caking, size and shape of particles, syringibility, and thixotropic nature of the suspensions. Recrystallization of drug particle should be done followed by its size reduction. All the ingredients should be sterilized prior to manufacturing. Manufacturing, filling and packing should all be carried out under aseptic conditions.

Suspensions can be aqueous or oily in nature and are usually administered by IM or SC route. The rate of drug release from suspension is based on factors such as dissolution of the drug, partition coefficient, blood perfusion, diffusion of drug through the adipose layer, depth of injection and composition of muscle and adipose tissues.

Some examples are chloramphenicol injection B.P. and methylprednisolone acetate injection B.P.