Pharmaceuticals can be roughly divided into three main groups:
- Pharmaceutical solutions: These are homogeneous mixtures which do not show sedimentation of particles. Solutions consist of a soluble material which is dissolved in a liquid/solvent and looks clear. A typical example is a sugar/water solution. Solutions can be further divided into aqueous solutions like douches, mouthwash, and nasal wash, non-aqueous solutions like elixirs, spirits, and collodions, and sweet or viscid solutions like cough syrup and honey.
- Pharmaceutical emulsions: These are biphasic systems consisting of two immiscible liquids. They are thermodynamically unstable and must be stabilized by the addition of an emulsifying agent. Examples are preparations for the treatment of skin conditions and for skin care. An advantage of emulsions compared to solutions is that they can deliver water-insoluble drugs via oral administrations.
- Pharmaceutical suspensions: These are heterogeneous mixtures containing solid particles (>1000 nm) that settle down. A good suspension must be stable during storage, should settle slowly, and re-disperse readily upon gentle shaking of the container. Drugs that have very low solubility are usually formulated as suspensions. A suspension may prolong the action of a drug by preventing rapid degradation in the presence of water, improve chemical stability of certain drugs (e.g. penicillin G), and mask an unpleasant taste of a drug.