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Routes of administration of a drug are determined by its physical and chemical properties, patient characteristics, and the rapidity of the desired response. The main routes are oral, parenteral and topical.
1. Oral route of administration is the safest, most cost-effective and most convenient way to administer drugs. Dosage forms for the oral route include tablets, capsules, powders, mixtures, emulsions, and gels. Most drugs are absorbed from the small intestine, but some are absorbed from the stomach and colon. In oral administration, the drug has a slow onset of action and a more prolonged but less potent effect when the drugs are given parenterally.
Some medications, such as nitroglycerin, can be given sublingually under the patient’s tongue, where they remain until dissolved and absorbed. The thin epithelium and rich capillary network under the tongue allow for rapid absorption and action of drugs. In addition, the drug is protected from hepatic inactivation and destruction by digestive enzymes as it reaches the general circulation without transversal through the liver.
2. Rectal administration may be convenient when the stomach is obstructed by vomiting, when the drug has an offensive taste or smell, or when it may 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 injection. These routes may be selected when the drug is poorly absorbed from the gut or is inactivated by digestive enzymes or metabolized during its passage through the liver or if the patient is unable to take or tolerate oral medication or if a rapid effect is desired. Medicines in solution are administered parenterally through a syringe-needle unit or an intravenous infusion set.
In intradermal injection, the drug is injected into the outer layers of the skin. The amount of drug is low and absorption is slow. This route is mainly used for diagnostic tests and is also used to inject the BCG vaccine.
Subcutaneous injections are made into the loose subcutaneous tissue beneath the skin. This route is used to inject small amounts of medication (2 ml or less). The absorption of the drug is slower and the effect of the drug is prolonged. This route is not recommended in the shock state because decreased peripheral circulation may decrease the absorption rate. Irritant drugs are not given subcutaneously because they are painful and can cause tissue necrosis. Hypodermoclysis is a type of subcutaneous injection that allows the slow administration of large volumes (500 – 1000 ml in adults) of fluids such as isotonic saline or glucose solutions. It is especially effective for infants and young children in preventing dehydration.
Intramuscular injections are given with a long and heavy needle that penetrates the subcutaneous tissue and the drug is deposited deep within the layers of the muscle mass. This route is suitable for administration of solutions and suspensions. Intravenous injections are used when an immediate drug effect is desired or when for some reason the drug cannot be injected into other tissues or when absorption may be impeded by poor circulation. This route is very valuable in emergency situations. The drug reaches the bloodstream directly.
An infusion is the intravenous administration of a larger volume of fluid, varying from 1-2 liters. The solution flows by gravity from the graduated bottle through a drip set. Infusions are given to relieve tissue dehydration, restore depleted blood volume, dilute toxins in the blood and tissues, provide electrolyte medications and food.
In intramedullary injections, the material is injected into the bone marrow of the sternum or tibia. The rapidity of the drug’s effect is comparable to intravenous injection. This route is used when veins are not available.
This highly specialized procedure involves placing a needle into an artery through an intra-arterial injection, whereby an arterial blood sample can be withdrawn for blood study or a radio-opaque substance is injected so that the segment’s artery is visible on an X-ray film. . The drug is injected intrathecally or intraspinally into the subarachnoid space. These injections are made by inserting a needle into the spinal fluid through the interspinous space of the spine, usually by lumbar puncture. This route is used to produce acute drug action in the cerebrospinal system or to produce spinal anesthesia.
Epidural injection of this drug is deposited through the vertebral interspace between the dura of the spinal cord and the periosteal lining of the spinal canal. In some emergencies, such as sudden cardiac arrest of an otherwise normal heart, intracardiac injections may be given. Injecting the drug directly into the heart may trigger a heartbeat.
4. Inhalation: Two classes of volatile and non-volatile substances can be administered by inhalation. Volatile substances such as gaseous anesthetics, vapors of liquid anesthetics, gases such as oxygen and carbon dioxide have a rapid effect when inhaled. Non-volatile substances break down into smaller particles and are inhaled as aerosols. They are so small that liquid or solid particles remain suspended for long periods of time instead of settling quickly due to gravity. The smaller the particle size, the deeper it reaches the respiratory tract. Common aerosol-producing devices are vaporizers, humidifiers, nebulizers, inhalers. Bronchodilators can be administered by inhalation for faster action.
5. Skin application: Absorption of drug through the skin is proportional to the dilute lipid solubility, because the epidermis lipid membrane acts as a barrier. The dermis is freely permeable to many fluids. Absorption through the skin can be enhanced by suspending the drug in an oily vehicle. Dosage forms are topically applied powders, liniments, creams, lotions, and jellies.
6. Mucosal applications are often used to produce systemic effects in the nose, throat, anus, vagina because of better absorption by the highly vascularized mucosa of these areas. Dosage forms 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. Enteral methods include oral, rectal, and sublingual. Parenteral methods include injection, inhalation, and application to the skin and mucous membranes.
