Routes of Administration

The route of administration is the path by which a drug is taken into the body, and it is one of the first determinants of how much drug reaches the systemic circulation and how quickly. Routes are broadly divided into enteral paths that use the gastrointestinal tract and parenteral paths that bypass it, with topical and inhalational routes alongside; each carries characteristic absorption and first-pass consequences.

Definition

A route of administration is the anatomical site and path through which a drug is introduced into the body, classified principally as enteral (via the gastrointestinal tract, e.g. oral, sublingual, rectal), parenteral (bypassing the gut, e.g. intravenous, intramuscular, subcutaneous), or topical and inhalational.

Scope

This topic covers the major routes by which drugs are administered, the enteral versus parenteral distinction, and how the chosen route shapes the rate and extent of absorption and exposure to first-pass metabolism. It treats routes as a pharmacokinetic concept and does not recommend a route for any condition.

Key concepts

• Enteral routes (oral, sublingual, rectal)
• Parenteral routes (intravenous, intramuscular, subcutaneous)
• Topical and transdermal administration
• Inhalational administration
• Bypassing first-pass metabolism
• Route-dependent rate and extent of absorption

Mechanisms

Each route exposes the drug to a different absorbing surface and a different sequence of barriers. Oral dosing requires dissolution and absorption across the gut wall and then transit through the liver, exposing the drug to first-pass metabolism; intravenous dosing places the drug directly into the blood, giving complete and immediate availability. Sublingual, rectal, transdermal, and inhalational routes can partly or wholly bypass the hepatic first pass, while intramuscular and subcutaneous routes deposit drug for absorption from tissue. The route therefore sets both the rate of entry and the degree of presystemic loss.

Clinical relevance

The route helps explain observed differences in onset and in the fraction of a dose that reaches the circulation, and understanding it is part of interpreting pharmacokinetic data. This entry is descriptive background on how route influences exposure; it is not guidance on selecting a route for any individual or condition.

Evidence & guidelines

Standard pharmacokinetic texts such as Rowland and Tozer set out how route governs absorption and first-pass exposure, and reviews of drug delivery such as Homayun and colleagues describe the barriers specific to particular routes, especially the oral route.

History

Classification of administration routes developed with pharmaceutics and clinical pharmacology as it became clear that the same agent behaves differently depending on its path into the body. The recognition that oral drugs are subject to first-pass elimination, synthesised by Pond and Tozer in 1984, sharpened the contrast between enteral and parenteral routes that organises the topic today.

Key figures

• Malcolm Rowland
• Thomas N. Tozer
• Susan M. Pond

Seminal works

• pond-tozer-1984
• homayun-2019

Frequently asked questions

What is the difference between enteral and parenteral routes?

Enteral routes deliver the drug through the gastrointestinal tract (for example oral or rectal), while parenteral routes bypass the gut and deliver the drug elsewhere, such as intravenously, intramuscularly, or subcutaneously.

Why can the route of administration change how much drug reaches the circulation?

Routes differ in the barriers a drug must cross and in whether the drug passes through the liver before reaching the systemic circulation; routes that avoid this first pass generally deliver a larger fraction of the dose intact.