Routes of administration of a drug are determined by its physical and chemical properties, patient characteristics and the rapidity of response desired. Major routes are oral, parenteral and topical.
1. The oral route of administration is the safest, most economical and the most convenient way of giving medicines. The dosage forms of the 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. On oral administration, 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 patient’s 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 inactivation and destruction by digestive enzymes as it reaches the general circulation without transverse through the liver.
2. Rectal administration can be advantageous when the stomach is retentive due to vomiting, when the drug has objectionable taste or odor or when it can be destroyed by digestive enzymes.
3. 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 gut or inactivated by digestive enzymes or metabolized during its passage through liver or if the 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 injecting 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 lesser). Drug absorption is slower and drug action is longer. This 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. Hypodermoclysis is a form of subcutaneous injection that permits the slow administration of large amounts (500 – 1000 ml in adults) of fluid such as isotonic saline or glucose solution. It is particularly useful in infants and young children to counteract dehydration.
Intramuscular injections are given with a 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 solution 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 emergencies. The drug reaches directly into the bloodstream.
An infusion is the intravenous administration of larger amounts of fluid, varying from 1- 2 litres. 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 electrolytes drugs and food.
In Intramedullary injections material is injected into the bone marrow of the sternum or tibia. Rapidity of drug effect is comparable with intravenous injection. This route is used when veins are not available.
Intra arterial injection in this highly specialized procedure the needle is placed in an artery, through which an arterial blood sample may be withdrawn for blood studies or a radio-opaque substance may be injected to make arteries of the part visible on an x-ray film. The drug is injected into the subarachnoid space in Intrathecal injections or intraspinal. These injections are made by inserting the needle through the vertebral interspinous spaces into the spinal fluid, usually by lumbar puncture. This route is used to produce intense action of drugs on cerebrospinal system or to produce spinal anaesthesia.
Epidural injection in this drug is deposited through vertebral interspace between the dura of spinal cord and the periosteal lining of spinal canal. In some emergencies like sudden cardiac arrest of an otherwise normal heart, Intra cardiac injections may be given. Injection of drug directly into the heart may start the heartbeat.
4. Inhalations: Two classes of substances may be administered by inhalation Volatile and Non-Volatile. Volatile substances like gaseous anaesthetics, vapours of liquid anaesthetics, 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 are liquid or solid particles so small that they remain suspended for a long time instead of settling down rapidly due to gravity. Smaller the particle size deeper it would reach in the respiratory passages. Common aerosol-producing devices are vaporizers, humidifiers, nebulizers, inhalers. Bronchodilators may be administered by inhalation for prompt action.
5. Dermal applications: Absorption of drug through skin is proportional to thin lipid solubility, as epidermis acts like lipid membrane barrier. The dermis is freely permeable to many fluids .Absorption through skin may be enhanced by suspending the drug in an oily vehicle. Dosage form applied topically are Powders, Liniments, Creams, Lotions and jellies.
6. Mucosal applications It is often used in the nose, throat, rectum, vag*ina to produce systemic effects because of good absorption through the highly vascularised mucosa of these areas. Dosage form for mucosal application include lozenges, sublingual tablets, suppositories, aerosols and nasal solutions.
At the end, we can conclude that there are Enteral and Parenteral routes of drug administration. The enteral method includes Oral, rectal and sublingual. Parenteral methods include injections, inhalations, and application to the skin and mucous membrane.
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