The administration of drugs via parenteral routes allows for direct drug introduction into the systemic circulation, resulting in high bioavailability because the medication bypasses the harsh conditions of the gastrointestinal tract and hepatic metabolism.

The intravenous route (IV) of drug administration can be further categorized into two types. The bolus injection administers the entire dose rapidly, while an intravenous infusion slowly delivers smaller doses steadily.

The IV route is often chosen for irritating solutions because the rapid blood flow dilutes the solution quickly, reducing potential discomfort or harm to the patient. However, it is important to note that IV injections can also lead to rapid and high drug concentrations in the bloodstream, increasing the risk of adverse reactions. Additionally, if the site of administration is contaminated, infectious agents can be introduced.

Another parenteral method is the intramuscular route, where the drug is injected directly into a muscle. Blood perfusion in muscles facilitates drug absorption. Absorption is usually fast for aqueous solutions of drugs, whereas nonaqueous drugs result in slower and prolonged absorption.

The subcutaneous route delivers the drug into the tissues between the skin and muscle layer. This method usually provides slow and steady absorption, resulting in a sustained effect over time. Certain medications are designed to be implanted in their solid forms under the skin, which allows for gradual absorption and extended therapeutic activity.