The term injection includes administration of intravenous (IV), intramuscular (IM), and subcutaneous (SC) areas.

The benefit of injections as a drug delivery form is that they act swiftly. The onset of action for IV is 15–30 seconds, IM is 10–20 minutes and SC is 15–30 minutes. Injections are also essentially bioavailable. They are helpful for drugs that are inefficient in their absorbtion or worse, do not show any result through oral administration.

Certain drugs, especially antipsychotics, are best delivered through long-acting IM injections. In process IV injectables can be helpful for delivery of regular drugs, even in the form of liquids. In some cases, patients are trained to use the injectables themselves. Injections required in SC areas to give insulin is patient-friendly. They can be managed without the involvement of highly trained profesionals.