Suspensions - Introduction to Suspensions

  1. Definition

    A suspension is a two phased system in which a finely divided solid is dispersed in a continuous phase of solid, liquid, or gas. The undissolved solid exists in equilibrium with a saturated solution of the solid in the continuous phase.

    A pharmaceutical suspension is a coarse dispersion in which insoluble solid particles are dispersed in a liquid medium.

  2. Size Classification of Suspensions

    A pharmaceutical suspension is a coarse dispersion in which insoluble solid particles are suspended in a liquid medium.

    A coarse suspension is a dispersion of finely divided, insoluble solid particles (the disperse phase) in a liquid (the dispersion medium). Most pharmaceutical suspensions consist of an aqueous dispersion medium although in some instances it may be an organic or oily liquid. A disperse phase with a mean diameter of up to 1 mm is usually termed a colloidal dispersion and include such examples as aluminum hydroxide and magnesium hydroxide suspensions.

    A solid in liquid dispersion in which the particles are above colloidal size is termed a coarse suspension.

  3. Routes of Administration

    • Oral
    • Parenteral
    • Topical (lotions)

  4. Forces In a Suspension

  5. Properties of a Well-Formulated Suspension

    The following animation introduces the properties of a well-formulated suspension.

    In Summary, Properties of an Ideal Suspension are:

    • Uniform dispersion
    • Palatable
    • Pleasing odour and colour
    • No grittiness
    • Easy to pour yet not watery
    • No cap-lock
    • Temperature insensitive

  6. Uses of Suspensions

    • To administer an insoluble compound as a liquid.
    • To lessen the unpleasant taste of an insoluble compound by:
      • formulating a vehicle in which the drug is not soluble
      • using an insoluble form of the drug (ie. salt form or prodrug)
      • adsorbing the drug onto an insoluble carrier
    • To modify the release rate of the drug.
    • To improve the stability by reducing the fraction of drug in solution.

  7. Suspensions as a Dosage Form

    Comparing suspensions with solutions:

    The solution is considered to be the most bioavilable liquid dosage form. Although also a liquid, the suspension has a dissolution step. However, relative to solid dosage forms, the suspension offers two advantages:

    • Drug particle size must be small to avoid a gritty mouth-feel.
    • All solid particles must be wetted to achieve physical stability.