Although an aqueous vehicle is generally preferred for an injection, it may be unacceptable if the drug has limited water solubility or is susceptible to hydrolysis. When such physical or chemical factors limit the use of an aqueous vehicle, then the only alternative is to use nonaqueous vehicles.

Examples of nonaqueous solvents employed in parenteral products are fixed vegetable oils, glycerin, polyethylene glycols, propylene glycol, alcohol, ethyl oleate, isopropyl myristate and dimethyl acetamide.

Characteristics of Nonaqueous Vehicles: A nonaqueous vehicle should have the following characteristics:

1. It must be nonirritating, nonsensitizing and nontoxic in the amounts administered.
2. It must not exert a pharmacological activity of its own.
3. It should not adversely affect the activity of the drug.
4. It should be physically and chemically stable at various pH levels.
5. Its viscosity must allow ease of injection and good syringibility.
6. Its fluidity must be maintained over a fairly wide temperature range.
7. Its boiling point should be sufficiently high to permit heat sterilization.
8. It should be miscible with body fluids.
9. It should have low vapor pressure to avoid problems during heat sterilization.
10. It should have constant purity or should be capable of purification.

Nonaqueous vehicles may be used provided they are safe in the amounts administered and do not interfere with the therapeutic efficacy of the preparation or with its response to prescribed assays and tests.

The USP specifies restrictions on the use of fixed vegetable oils in parenteral products. The oils used must remain clear when cooled to 10°C to ensure the stability and clarity of the product during refrigeration. They must not contain mineral oil or paraffin, as these materials are not absorbed by body tissues.

Examples of fixed oils used in injections are corn oil, cottonseed oil, peanut oil and sesame oil.