Suspensions as defined by the
physical chemist are a two – phase system consisting of a finely divided solid
dispersed in a solid, liquid or gas. Pharmacist accepts the definition and
admits that a variety of a dosage form fall within the scope such as Gels,
Lotions, Magmas, Mixtures and the official Suspensions. In general sense each
of these preparations represents a suspension but state of subdivision of the
insoluble solid varies from particles which gradually subside on standing to
particles which are colloidal in nature. The lower limit of particles size is
approximately 0.1 micron and it is the preparations containing dispersed solid
of these magnitude or greater that are pharmaceutically defined as suspensions.
Suspension formulation and control is based on the principle of Colloidal
Dispersion and on Rheology. Formulation involves more than suspending a solid
in a liquid. Knowledge of the behavior of particles in liquids, of suspending
agents and of flavors and colors is required to produce a satisfactory
suspension.
Advantages of suspension over other dosage
forms:
1. Some drugs are insoluble in
all acceptable media and therefore administered as a tablet, capsule, etc. or
as a suspension.
2. The liquid character insures
some uniformity of dosage.
3. Disagreeable taste can be
masked by the use of a suspension of the drug or a derivative of the drug,
example of the latter, Chloramphenicol Palmitate.
4. More stable than solutions.
5. Ideal dosage – form for
patients who have difficulty of swallowing tablets and capsules.
Basic properties or factors of
a good suspension:
1. Dispersed phase should
settle slowly and should be readily redispersed on shaking.
2. Should not cake on
settling.
3. Viscosity should be such
that preparation pours easily.
4. Should be acceptable to the
patient on the basis of its taste, color, and cosmetic qualities. The last two
being particularly important in the preparation intended for external use.
Official Suspensions
Preparations of finely divided
drugs, either intended for suspension in some suitable liquid vehicle prior to
use or already in suspension in a liquid vehicle, either for oral, ophthalmic,
and parenteral use.
Example of those already
suspended for oral use is the Trisulfapyrimidine Oral Suspension – three sulfapyrimidine
are already suspended in a liquid, flavored vehicle in a form suitable for oral
use.
Example of those suspended in
liquid vehicle prior to use is the Tetracycline for Oral Suspension. It is a
finely divided tetracycline mixed with suspending and dispersing agents
intended to be diluted with the prescribed volumes of purified water and mixed
before it is dispensed by the pharmacist for oral administration to the
patient.
The particulate matter of a
suspension tends to settle slowly form the liquid vehicle in which it is
dispersed, since its density is almost always greater than that of the liquid
vehicle. In some cased, an added inert suspending agent is permitted to retard
the sedimentation, to increase vehicle density, viscosity, or yield value.
Suspensions of solids in the vehicle and thereby gives a uniform and proper
dosage. Suitable preservatives should be added to protect the preparation form
bacterial and mold contamination.
