intramuscular administration

Intramuscular (IM) administration injects a drug into a large muscle — commonly the deltoid in the upper arm, the thigh, or the buttock. Muscle is richly supplied with blood vessels, so a drug deposited there is picked up and carried into the circulation fairly quickly, though not as instantly as an intravenous dose.

Absorption from muscle is generally faster than from under the skin and slower than from a vein, because muscle has plentiful blood flow but the drug must still diffuse from the injection depot into capillaries. The rate can be tuned by formulation: a water-based solution absorbs quickly, while an oily or crystalline 'depot' preparation dissolves slowly to release drug over days, weeks, or even months — useful for long-acting contraceptives and antipsychotics.

IM is useful when a drug cannot be given orally and an immediate IV effect is not required, or when no vein is accessible. It can accommodate moderate volumes (typically up to about 2–5 mL depending on the muscle) and some irritant or poorly soluble drugs.

Limitations include pain at the site, possible nerve injury if poorly placed, and the fact that absorption can be erratic in shock or poor circulation (when muscle blood flow falls). IM injections should be avoided in patients on anticoagulants because of bleeding risk into the muscle.