01.08.4 Intramuscular

Intramuscular is injection into the muscle. An advantage of intramuscular (i.m.) administration is that it avoids first pass liver metabolism, and it may be suitable for drugs that undergo extensive first pass liver metabolism. The intramuscular route is often suitable for drugs that are mucosal irritants or drugs that are irritants after subcutaneous injection.

Drugs in aqueous solution are absorbed quite rapidly from muscle, but the rate of absorption will depend on the rate of blood flow at the injection site. If the blood flow is high, the rate of absorption will also be high. The rate of absorption of an aqueous solution is greater from the deltoid (shoulder muscle) or vastus lateralis (above the knee muscle) than from the gluteus maximus (butt muscle). Absorption from gluteus maximus is particularly slow in females who have more subcutaneous fat in this area than males, because fat slows the absorption. Gardasil, the vaccine designed to prevent most cases of cervical cancer, is injected into the deltroid for quick absorption.

Blood flow to the legs is increased during jogging. If insulin is injected into the leg, rather that into the arm or abdominal wall, as recommended, it is absorbed quickly during jogging. With higher levels of insulin than expected, there is an increased take up of glucose into the tissues, which can cause an unwanted major drop in blood glucose leading to an episode of hypoglycaemia.