Routes of Drug Administration: An Overview

The route of drug administration is can be defined as the route by which a drug is delivered into the body for diagnosis, prevention, cure, or treatment of various diseases and disorders. For a drug to perform its desired therapeutic effect, it must come in contact with the tissues of organs and cells of tissues by one way or the other, and for this to take place the drug must be administered in the appropriate way.

The route of administration of medication directly affects the drug bioavailability, which determines both the onset and the duration of the pharmacological effect. The choice of route of administration may be influenced by many factors among which include:

Convenience
State of the patient
The desired onset of action
Patient’s co-operation
The nature of the drug as some drugs may be effective by one route only e.g., Insulin
Age of the patient
Effect of gastric pH, digestive enzymes and first-pass metabolism

The various routes of administrations are classified into the local routes and systemic routes. The local route is the simplest mode of administration of a drug at the site where the desired action is required. When the systemic absorption of a drug is desired, medications are usually administered by two main routes: the enteral route and the parenteral route.

Enteral route involves the absorption of the drug via the gastrointestinal tract and includes oral, sublingual, and rectal administration.
The parenteral route, on the other hand, refers to any routes of administration that do not involve drug absorption via the gastrointestinal tract (par = around, enteral = gastrointestinal), including injection routes (e.g., intravenous route, intramuscular route, subcutaneous route, etc.), inhalational and transdermal routes.

Different routes of drug administration

The main routes of drug administration include:

Oral route
Sublingual/ Buccal route
Rectal route
Topical route
Transdermal route
Inhalational route/ pulmonary route
Injection route

1. Oral route

This is the most frequently used route for drug administration. When possible, it is the first choice for the administration of drugs, since it is both convenient and economical. Drugs administered orally are placed in the mouth and swallowed.
Most drugs that are given orally are absorbed into the circulation from the gastrointestinal tract very efficiently within the limits of the physicochemical properties of the drug concerned. Certain drugs are taken orally for their local effects within the bowel e.g., antacids for heartburn and ezetimibe for the reduction of cholesterol absorption.
Oral agents must be able to withstand the acidic environment of the stomach and must permeate the gut lining before entering the bloodstream. The most popular oral dosage forms are tablets, capsules, suspensions, solutions, and emulsions.

2. Sublingual/ Buccal route

In this route of administration, the drug is placed under the tongue (sublingual route) or between gums and inner lining of the cheek (buccal route). In both cases, the drug is allowed to dissolve, avoiding swallowing as far as possible. The drug is rapidly absorbed through the mucosa into circulation, thereby bypassing the portal circulation and, thus, the first-pass metabolism in the liver.
Sublingual and buccal routes are of value when the medication concerned is destroyed or partially inactivated in the stomach if swallowed and when more rapid action is required. These routes however are not suitable for bitter preparations.
Examples of drugs administered through sublingual and buccal routes are nitro-glycerine (glyceryl trinitrate), buprenorphine, and des amino-oxytocin.

3. Rectal route

Medications are sometimes ordered to be administered by the rectal route. The rectal mucosa is capable of absorbing many soluble drugs into the circulation. Rectal medication may be in suppository form or in liquid form to be administered as a retention enema.
Unlike the oral route, drugs with an irritant or unpalatable nature can be administered through the rectum. Rectal route can also be preferred when the patient has persistent vomiting or is unable to swallow. Also, this route can be used for systemic drug administration in addition to the local administration.

4. Topical route

Drugs are applied topically, that is to the skin or mucous membrane of the eye, ear, nose, mouth, vagina, etc., mainly for local action. This route provides a high local concentration of the drug without affecting the general circulation. However, drugs that are absorbed into the circulation after local administration may then have systemic effects. Drugs for topical applications are usually available as creams, ointment, liniment and drops.

5. Transdermal route

The transdermal route is commonly referred to as “the patch” because the medication is contained in a patch that is absorbed through the skin. Drugs administered through this route must be highly lipophilic. Absorption via this route is slow but conducive to producing long-lasting effects. Special slow-release matrices in some transdermal patches can maintain steady drug concentrations that approach those of constant IV infusion. Transdermal patches also provide fewer absorption problems in the gastrointestinal tract that are commonly experienced by patients who take oral medications.
Drugs administered through this route include fentanyl patches for severe pain management, nitroglycerin transdermal patch used to prevent episodes of angina in people who have coronary artery disease, nicotine patches for the cessation of smoking, etc.

6. Inhalational route/ pulmonary route

Drug delivery by inhalation is a common route, both for local and for systemic actions. This delivery route is particularly useful for the direct treatment of asthmatic problems, using both powder aerosols (e.g. salmeterol xinafoate) and pressurized metered-dose aerosols containing the drug in liquefied inert propellant (e.g. salbutamol sulfate inhaler).
Drugs may be inhaled as gases (e.g., nitrous oxide) and enter the bloodstream by diffusing across the alveolar membrane. This is the method of administration of volatile anesthetics such as ether, halothane, and methoxyflurane.
The lungs provide an excellent surface for absorption when the drug is delivered in gaseous, aerosol mist or ultra-fine solid particle form. This results in rapid onset of action. Another advantage is that plasma concentration can be rapidly adjusted as well.

7. Injection routes

This is the second most popular route of drug administration. They mainly involve introducing the drug in the form of solution or suspension into the body at various sites and to varying depths using syringe and needle. Thus administration involves the risk of infection, pain, and local irritation.
Injection routes of drug administration are usually employed where:

The rapid effect is urgently needed as in emergency situations;
The patient is too ill or unconscious for the oral route to be employed;
The drug is orally ineffective due to its being destroyed or not absorbed from the gut;
An injection is the only way for the drug to reach its required site of action;
There is a need to maintain a steady blood level of a drug.

The most important factors or requirements in all injection routes are the surrounding tissue or site must be as clean as possible, and all instruments used must be clean and sterile. There are four commonly used injection routes: intradermal (ID), subcutaneous (SC), intramuscular (IM), and intravenous (IV). Other routes such as intra-arterial (IA), intrathecal (IT), intraperitoneal (IP), intravitreal etc., are used less frequently.

Injection routes - Definition

Subcutaneous (SC) - Administration beneath the skin; hypodermic. Synonymous with the term subdermal or hypodermal.
Intramuscular (IM) - Administration within a muscle.
Intradermal (ID) - Administration within the dermis.
Intravenous (IV) - Administration within or into a vein or veins.
Intra-arterial (IA) - Administration within an artery or arteries.
Intrathecal (IT) - Administration within the cerebrospinal fluid at any level of the cerebrospinal axis, including injection into the cerebral ventricles
Intraperitoneal (IP) - Administration within the peritoneal cavity.
Intravitreal - Administration within the vitreous body of the eye.