Routes of drug administration

The route of administration, absorption site, and bioavailability of the drug from the dosage form are important factors in the manufacturing of the drug product.

Drugs intended for local activity, such as topical antibiotics, anti-infectives, antifungal agents and local anesthetics are the formulations in dosage form that minimize systemic absorption. The concentration of these drugs at the application site affects their activity.

When systemic absorption is desired, bioavailability of the drug from the dosage form at the absorption site must be considered (e.g., a drug given intravenously is 100% bioavailable because all of the drug is administered directly into the systemic circulation). The amount of the dose depends on the drug absorption and the desired systemic drug concentration. The type of dosage form (e.g., immediate release or controlled release) affects the rate of drug absorption.

There are different forms of parenteral drug administration, including intravenous and intra-arterial injection, intravenous infusion, intramuscular injection, subcutaneous injection, intra-articular injection, intradermal injection, and intratechal injection. In intra-arterial injection the drug is injected into a particular artery to achieve a high drug concentration in a specific tissue before drug distribution throughout the body occurs. Intra-arterial injection is administered for diagnostic purposes, and occasionally in chemotherapy.

Enteral administration includes the buccal and sublingual routes, and oral and rectal administration. In buccal and sublingual administration, the drug is absorbed directly into the systemic circulation, bypassing the liver. When the drug is administered orally, it is swallowed, and undergoes absorption from the gastrointestinal tract through the mesenteric circulation into the liver and then to the systemic circulation. Oral route is the most common, as well a as the most convenient, and safest route of drug administration, though it has a number of disadvantages.

In rectal administration, the drug in the form of solution (enema) or in semisolid form (suppository) is placed into the rectum. It is absorbed through the mucosa in the lower two thirds of the rectum, and enters the systemic circulation directly, bypassing the liver.

Respiratory tract administration involves intranasal administration and pulmonary inhalation. Intranasal administration may be used for local, or systemic effects. The drug contained in a solution or suspension is administered to the nasal mucosa either as a spray or as drops. In pulmonary inhalation, the drug is inhaled from a nebulizer or an aerosol container into the respiratory tract.

Transdermal (percutaneous) drug absorption is application of the drug (in the form of a lotion, cream, paste, or patch) on skin surface for systemic absorption. An occlusive dressing or film improves systemic drug absorption from the skin. Such drugs as antibacterial agents, and local anesthetics are applied topically onto the skin for a topical effect.

Other routes of drug administration include ophthalmic, otic, urethral, and vaginal administration. These routes are generally used for achieving a local therapeutic effect. However, some systemic drug absorption may also occur.