A bolus injection is the administration of drug(s) in a bolus. A bolus is a single, large dose of a drug. Bolus injections become necessary when a patient needs a particular medication(s) immediately circulating in the bloodstream. For instance, a patient with high fever due to infection or microbial disease would need a high volume of antibiotic in the bloodstream in order to start working quickly. The antimicrobial action of the antibiotic is necessary so that the disease and high fever of the patient would be managed or controlled in no time. This method of drug administration is usually followed by the administration of a longer-term, slower-acting intravenous drip line of medication.
Bolus injections may be administered through these routes: intravenous, intramuscular, and intrathecal. An intravenous bolus injection is one that is administered intravenously as exemplified above. Chemotherapy is another example. An intramuscular bolus injection is the administration of a drug bolus into a muscle. An example is the administration of insulin subcutaneously. An intrathecal bolus injection is the administration of drug under the arachnoid membrane of the brain or spinal cord, such as the administration of anesthesia prior to a CS section.