vehicle

A vehicle is the inert carrier — usually a liquid — in which a drug is dissolved or suspended so it can be measured, swallowed, injected or applied. If the drug is the passenger, the vehicle is the car that carries it. Water is the most common vehicle, but oils, syrups and oily bases are also used.

The vehicle does no therapeutic work itself, yet its choice matters greatly. It must dissolve or evenly disperse the drug, keep it stable, be safe by the intended route, and feel acceptable. A drug that will not dissolve in water may need an oily vehicle, a cosolvent such as ethanol, or a surfactant to be carried at all.

Vehicle choice is constrained by route. An injection vehicle must be sterile and, for intravenous use, water-miscible and non-irritant; a topical vehicle can be an oily or aqueous base chosen for feel and skin penetration. The same drug may sit in quite different vehicles across its dosage forms.