Routes of drug administration

Routes of administration of a drug are determined by its physical and chemical properties, patient’s characteristics and the rapidity of response desired. Major routes are oral, parenteral and topical.

Oral route of administration is the safest, most economical and the most convenient way of giving medicines. The dosage forms of oral route include tablets, capsules, powders, mixtures, emulsions and gels. Most drugs are absorbed from small intestine but some are absorbed from stomach and colon. An oral administration of drug action has a slower onset and more prolonged but less potent effect than when drugs are given parenterally. Some drugs like nitroglycerin may be given sublingually by placing them under the tongue, where they are retained until dissolved and absorbed. The thin epithelium and the rich capillary network under the tongue permit rapid absorption and drug action. In addition, the drug is saved from hepatic inactivationand destruction by digestive enzymes as it reaches the systemic circulation without passing through the liver.

Rectal administration can be preferable when the stomach is quick on the uptake due to vomiting, when the drug has bad taste or odor, or when it can be destroyed by digestive enzymes.

Parenteral route refers to any route other than gastrointestinal, but is commonly used to indicate subcutaneous, intramuscular and intravenous injections. These routes may be selected when the drug is poorly absorbed from the intestine, or inactivated by gastric enzymes, or metabolized during its passage through the liver, or if a patient is unable to take or tolerate oral medication, or if a rapid effect is desired. Drugs in solution are administered parenterally by means of a syringe – needle unit or an intravenous infusion set.

In intradermal injections the drug is injected into the outer layers of the skin. The amount of drug is small and absorption is slow. This route is mainly used for diagnostic tests and it is also used for BCG vaccine.

Subcutaneous injections are made into the loose subcutaneous tissue under the skin. This route is used to inject small amount of drug (2 ml or less). Drug absorption is slower - drug action is longer. The route is not advised in shock states because the reduced peripheral circulation may decrease the absorption rate. Irritant drugs are not given subcutaneously as they are painful and may cause tissue necrosis.

Intramuscular injections are given with longer and heavier needle that penetrates the subcutaneous tissues, and the drug is deposited deep between the layers of muscle mass. This route is suitable for administration of solutions and suspensions.

When an immediate drug effect is desired, or when for any reason the drug cannot be injected into the other tissues, or when absorption may be inhibited by poor circulation, intravenous injections are given. This route is of great value in case of emergency. The drug reaches directly into the bloodstream. An infusion is the intravenous administration of larger amounts of fluid up to 1-2 liters. The solution flows by gravity from a graduated bottle through a drip set. Infusions are mostly given to relieve tissue dehydration, to restore depleted blood volume, to dilute the toxic substances in the blood and tissues, to supply electrolyte drugs and food.

Two classes of substances may be administered by inhalation, volatile and non-volatile. Volatile substances like gaseous anesthetics, vapors of liquid anesthetics, gases like oxygen and carbon dioxide produce rapid effects when inhaled. Non-volatile substances have to be broken down into small particles and inhaled as aerosols. These liquid or solid particles are so small that they remain suspended for a long time instead of rapid deposition due to the gravity. Smaller the particle size, deeper it would reach in the respiratory passages.

Dermal application. Absorption of a drug through the skin is proportional to thin lipid solubility, as epidermis acts like lipid membrane barrier. The dermis is freely permeable to many fluids. Absorption through the skin may be enhanced by suspending the drug in an oily vehicle. Dosage forms applied topically are powders, liniments, ointments, creams, lotion and gels.

Mucosal application is often used in the throat, nose, rectum and vagina to produce systemic effect because of good absorption through highly vascularised mucosa of these areas. Dosage forms for mucosal application include lozenges, sublingual tablets, suppositories, aerosols and nasal solutions.

So, we can conclude that there are enteral and parenteral routes of administration. The enteral method includes oral, rectal and sublingual. The parenteral method includes injections, inhalations and applications to the skin and mucous membranes.

VOCABULARY PRACTICE