The administration of drugs is an essential aspect of medical practice. It involves the process of delivering medication to a patient’s body through various routes. The appropriate route of administration for a given situation depends on several factors, including the drug’s physical and chemical properties and patient-related factors. This chapter discusses the different routes of drug administration and factors governing the choice of route.
Factors Governing Choice of Route Several factors govern the choice of route for drug administration. These factors include:
- Physical and Chemical Properties: of the Drug The physical and chemical properties of the drug, such as solubility, stability, pH, and irritancy, determine the route of administration. For example, drugs that are solids, liquids, or gases may require different routes of administration.
- Site of Desired Action: The site of the desired action can be localized or generalized. Localized sites can be approachable, such as the skin, eyes, or ear canal, while generalized sites may not be approachable, such as deep tissues.
- Rate and Extent of Absorption: The rate and extent of drug absorption from different routes are essential considerations. The absorption rate varies depending on the route of administration, with some routes offering faster and more extensive absorption than others.
- Effect of Digestive Juices and First-Pass Metabolism: The effect of digestive juices and first-pass metabolism on the drug also influences the route of administration. Oral medications, for example, may be subject to first-pass metabolism, resulting in lower bioavailability.
- Rapidity of Response: The rapidity with which a response is desired also plays a role in the choice of route. In emergency situations, for example, intravenous administration may be necessary for a rapid response.
- Accuracy of Dosage Required: The accuracy of the dosage required may also dictate the route of administration. Intravenous and inhalational routes provide fine-tuning for precise dosages.
- Patient Condition: The patient’s condition, such as unconsciousness or vomiting, may limit the routes of administration available.
Local Routes of Administration
Local routes of administration are used for localized lesions at accessible sites and for drugs whose systemic absorption from these sites is minimal or absent. High concentrations are attained at the desired site without exposing the rest of the body to systemic side effects or toxicity. The following are the local routes of administration:
- Topical Administration: Refers to the external application of drugs to the surface for localized action. It is often more convenient and reassuring for the patient. Drugs can be efficiently delivered to localized lesions on the skin, oropharyngeal/nasal mucosa, eyes, ear canal, anal canal, or vagina in the form of lotion, ointment, cream, powder, rinse, paints, drops, spray, lozenges, suppositories, or pessaries. Nonabsorbable drugs given orally for action on the gastrointestinal mucosa, inhalation of drugs for action on bronchi, and irrigating solutions/jellies applied to the urethra are other forms of topical medication.
- Deeper Tissue: Certain deep areas can be approached using a syringe and needle, but the drug should be in such a form that systemic absorption is slow. For example, intra-articular injection of hydrocortisone acetate in the knee joint, infiltration around a nerve or intrathecal injection of lidocaine, and retrobulbar injection of hydrocortisone acetate behind the eyeball.
- Arterial Supply: Close intra-arterial injection is used for contrast media in angiography. Anticancer drugs can be infused in the femoral or brachial artery to localize the effect for limb malignancies.
Systemic Routes of Drug Administration
Drug administration through systemic routes is a crucial aspect of modern medicine, as it aims to absorb drugs into the bloodstream and distribute them throughout the body. This chapter will discuss the different systemic routes of drug administration, their advantages, and their limitations.
Oral administration is the oldest and most commonly used route of drug administration. This route is safe, convenient, and non-invasive. It is suitable for both solid and liquid dosage forms. However, oral administration has limitations such as slow action, unsuitability for emergencies, difficulty in administering unpalatable drugs, nausea and vomiting, and variable absorption.
Sublingual (SL) and Buccal Administration:
Sublingual and buccal administration involves placing a tablet or pellet containing the drug under the tongue, crushing it in the mouth, and spreading it over the buccal mucosa. This method is suitable only for lipid-soluble and non-irritating drugs. The liver is bypassed, and drugs with high first-pass metabolism can be absorbed directly into the systemic circulation. Sublingual administration is suitable for drugs such as GTN, buprenorphine, and desamino-oxytocin.
Rectal administration is used for irritant and unpleasant drugs that can be put into the rectum as suppositories or retention enemas for systemic effect. This route is suitable for patients with recurrent vomiting or unconsciousness. However, it is inconvenient and embarrassing, absorption is slower, irregular, and unpredictable. Diazepam solution and paracetamol suppository are rapidly and dependably absorbed from the rectum in children.
Highly lipid-soluble drugs can be applied over the skin for slow and prolonged absorption. The liver is bypassed, and the drug can be incorporated into an ointment and applied over a specified area of skin. Transdermal therapeutic systems (TTS) are adhesive patches that deliver the contained drug at a constant rate into systemic circulation via the stratum corneum. TTS provides smooth plasma concentrations of the drug without fluctuations and minimizes interindividual variations and side effects.
Volatile liquids and gases are given by inhalation for systemic action, e.g., general anaesthetics. Absorption occurs from the vast alveoli’s vast surface, and the action is very rapid.
The mucous membrane of the nose can readily absorb many drugs, and digestive juices and liver are bypassed. However, only certain drugs like GnRH agonists, calcitonin, and desmopressin are suitable for this route.
Parenteral administration refers to the administration of drugs through injection, which directly delivers the drug into the tissue fluid or bloodstream without crossing the intestinal mucosa. This route of administration has several advantages over oral administration, such as faster and surer drug action, avoidance of gastric irritation and vomiting, and the ability to administer drugs to unconscious, uncooperative, or vomiting patients. The liver is bypassed, and there is no interference by food or digestive juices. However, parenteral administration also has some disadvantages, including the need for sterile preparations, higher cost, invasiveness, and the potential for local tissue injury.
Subcutaneous (SC) Administration:
Subcutaneous administration involves the injection of drugs into the loose subcutaneous tissue, which is richly supplied by nerves but less vascular than muscle tissue. This route is suitable for injecting small volumes of drugs, and self-injection is possible because deep penetration is not required. Depot preparations that are aqueous suspensions can be injected for prolonged action. Special forms of this route include the Dermojet method, which involves projecting a high-velocity jet of drug solution into the subcutaneous tissue using a gun-like implement, and pellet implantation, which involves introducing a solid drug pellet with a trocar and cannula for sustained release of the drug over weeks or months. Sialistic and biodegradable implants can also be used to achieve constant blood levels over months.
Intramuscular (IM) Administration:
Intramuscular administration involves the injection of drugs into one of the large skeletal muscles, such as the deltoid, triceps, gluteus maximus, or rectus femoris. Muscle tissue is less richly supplied with sensory nerves and more vascular than subcutaneous tissue, making the absorption of drugs in an aqueous solution faster. Depot preparations, such as oily solutions and aqueous suspensions, can also be injected using this route. However, self-injection is often impracticable because deep penetration is required, and this route should be avoided in anticoagulant-treated patients due to the risk of local hematoma.
Intravenous (IV) Administration:
Intravenous administration involves the injection of drugs as a bolus or infusion into one of the superficial veins. This route allows the drug to reach directly into the bloodstream, producing immediate effects, which is highly valuable in emergencies. The intima of veins is insensitive, and highly irritant drugs can be injected intravenously, although thrombophlebitis of the injected vein and necrosis of adjoining tissues are potential hazards if extravasation occurs. Diluting the drug or injecting it into a running intravenous line can minimize these complications. Only aqueous solutions should be injected intravenously, and there are no depot preparations for this route. This route carries the highest risk, as vital organs such as the heart and brain are exposed to high concentrations of the drug.
Intradermal administration involves the injection of drugs into the skin, raising a bleb, or scarring/multiple punctures of the epidermis through a drop of the drug. This route is employed for specific purposes only, such as the BCG vaccine and sensitivity testing.
Parenteral administration provides several advantages over oral administration, including faster and surer drug action and the ability to administer drugs to unconscious, uncooperative, or vomiting patients. However, this route also carries higher risks, such as invasiveness and the potential for local tissue injury. The subcutaneous, intramuscular, intravenous, and intradermal routes are commonly used for the parenteral administration of drugs, with each route having its own advantages and disadvantages.
Disclaimer: This article is for informational purposes only and should not be taken as medical advice. Always consult with a healthcare professional before making any decisions related to medication or treatment.