Intramuscular injections and bioavailability

Bioavailability of drugs following intramuscular injection is reviewed, with particular emphasis on diazepam, chlordiazepoxide, phenytoin, digoxin and lidocaine. Clinical experience with these drugs has shown that i.m. absorption may be slow, erratic or incomplete. Factors which play a role in the bioavailability of i.m. medications include the water solubility of the drug, dispersion of the injected solution and blood flow at the muscle site. For many drugs, intravenous injection is the parenteral route of choice, and oral administration may be more efficacious than i.m. injection.