According to the United States Pharmacopoeia (USP), injectable preparations are classified into five general types:

1. Injection: Solutions containing drug substances (e.g., Insulin injection USP)
2. For Injection: Dry solids that upon reconstitution with suitable vehicles give solutions and comply with the requirements for injections (e.g., Cefuroxime for injection USP)
3. Injectable Emulsion: Drug substances that are dissolved or dispersed in a suitable emulsion medium (e.g., Propofol USP)
4. Injectable Suspension: Solids suspended in a suitable liquid medium (e.g., Methylprednisolone acetate suspension USP)
5. For Injectable Suspension: Dry solids that upon reconstitution with suitable vehicle give a product that complies with the requirements for injectable suspensions (e.g., Imipenem, meropenem and cilastatin for injectable suspension USP)

The nature of the drug, with respect to certain therapeutic considerations and its physical and chemical properties, decides the form in which the drug is prepared for parenteral use by the manufacturer.

If a drug is unstable in solution, it can be prepared as a dry powder, which is to be reconstituted with the appropriate solvent during the time of administration. For drugs insoluble in water, the injection can be prepared using a suitable nonaqueous solvent, such as vegetable oil or may be prepared as a solution or an aqueous suspension. If an aqueous solution is desired, a water-soluble salt form of the insoluble drug is prepared. Aqueous or blood-miscible solutions may be injected directly into the blood stream. The use of liquids, such as oleaginous injections and suspensions, which are miscible with blood and can interrupt the normal flow of blood, is generally restricted to other than intravenous administration. The onset and duration of action of a drug may be controlled by its chemical form, the physical state of the injection vehicle used and the route of injection administration.