clinical pharmacology
Drug Absorption
Parenteral Administration
Drugs given IV enter the systemic circulation directly. However, drugs injected IM or subcutaneously must cross one or more biologic membranes to reach the systemic circulation. If protein drugs with a molecular mass>20,000 g/mol are injected IM or sc, movement across capillary membranes is so slow that most absorption occurs via the lymphatic system. In such cases, drug delivery to systemic circulation is slow and often incomplete because of first-pass metabolism (metabolism of a drug before it reaches systemic circulation) by proteolytic enzymes in the lymphatics. Perfusion (blood flow/gram of tissue) greatly affects capillary absorption of small molecules injected IM or sc. Thus, injection site can affect absorption rate. Absorption after IM or subcutaneous injection may be delayed or erratic for salts of poorly soluble bases and acids (eg, parenteral form ofphenytoin) and in patients with poor peripheral perfusion (eg, during hypotension or shock). 