01.08.3 Subcutaneous

Subcutaneous is injection of a drug under the skin. An advantage of subcutaneous (s.c.) administration is that it avoids first pass liver metabolism, as the drug is released into the circulation, rather than specifically into the portal vein. From the subcutaneous area, absorption is often by diffusion. The drug is administered in a depot under the skin with absorption into plasma, and then to the circulation often sufficiently constant and slow to provide a sustained effect. Thus, subcutaneous administration can provide a slow release preparation for ongoing effects. Also, the rate of absorption after subcutaneous administration can be varied. For instance, in diabetics, soluble insulin with rapid absorption can be injected subcutaneously to mimic the rapid rise in blood insulin observed with a meal. Whereas an insoluble suspension of insulin can be used for the slow absorption required to give the low constant fasting level of insulin.

The area affected by the subcutaneous administration of a drug can be limited by the injection of a vasoconstrictor (e.g. adrenaline) with the drug. The localised vasoconstriction response to adrenaline limits the transport of the drug away from the site of injection in the circulation.

This method is used with local anaesthetics to keep the effect of the anaesthetic confined to a local area. Without a vasoconstrictor, the local anaesthetic and the anaesthetic effects spread from the site of injection, and are no longer local.