VPT 311: General and Systemic Veterinary Pharmacology (2+1)

ABSORPTION OF DRUGS AFTER PARENTERAL ADMINISTRATION

• Drugs injected intramuscularly or subcutaneously may readily diffuse through tissue fluid and reach a capillary to be absorbed. Anything that interferes with diffusion of the drug from the site of administration or alters the blood flow to the injection site can delay absorption of the drug. The major factor that determines the absorption is the blood flow to the muscle.
• Aqueous solutions of drugs are usually absorbed from intramuscular injection site within 10-30 minutes provided the blood flow is unimpaired. Faster or slower absorption is possible, depending on the concentration and lipid solubility of the drug, vascularity of the site, volume of injection, the osmolality of the solution and other pharmaceutical factors.
• Absorption of drugs from subcutaneous tissues is influenced by the same factors that determine the rate of absorption from intramuscular sites. Some drugs are absorbed as rapidly from subcutaneous tissues as from muscles, although absorption from injection sites in subcutaneous fat is always significantly delayed.
• Increasing the blood supply to the injection site by heating, massage or exercise hastens the rate of absorption. Spreading and absorption of a large fluid volume, which has been injected subcutaneously may be facilitated by including hyaluronidase in the solution.