There are several routes of administration for medications, which refer to the ways in which medications can be introduced into the body. The choice of route depends on factors such as the properties of the medication, the desired therapeutic effect, the patient’s condition and the medications’s intended target site.
Routes of administration
Common routes of administration include:
- Oral: Medications are taken by mouth, usually in the form of tablets, capsules, or liquids. They are swallowed and absorbed through the gastrointestinal tract.
- Inhalation: Medications are delivered through inhalers or nebulisers and inhaled into the respiratory tract. This route is often used for treating respiratory conditions such as asthma or chronic obstructive pulmonary disease (COPD).
- Injection: Medications are delivered directly into the body using a needle and syringe. There are different types of injections:
- Intravenous (IV): Medications are injected into a vein, allowing for rapid absorption and distribution throughout the body.
- Intramuscular (IM): Medications are injected into a muscle, where they are absorbed more slowly than IV injections but generally more quickly than oral administration.
- Subcutaneous (SC): Medications are injected into the fatty tissue just beneath the skin. This route provides slower and sustained absorption.
- Intradermal (ID): Medications are injected into the dermis layer of the skin, typically used for diagnostic tests such as tuberculin skin tests.
- Intraosseous (IO) access is typically established in the proximal tibia (shinbone) or distal humerus (upper arm bone) using specialised IO needles or devices. These devices penetrate the bone cortex and allow for the infusion of fluids, medications or blood products.
- Transdermal: Medications are delivered through patches that adhere to the skin. They slowly release the medication, allowing for absorption through the skin into the bloodstream.
Pharmacokinetics
The overall process involving absorption, distribution, metabolism and elimination of medications in the body is referred to as pharmacokinetics. Pharmacokinetics is the study of how drugs are absorbed, distributed, metabolised and eliminated by the body. It focuses on understanding the time course and extent of drug concentration changes in various compartments of the body over time.
Medications typically pass through several stages in the body, including absorption, distribution, metabolism, and elimination.
- Absorption: Absorption refers to the process by which medications enter the bloodstream from the site of administration. The absorption rate and extent depend on factors such as the route of administration, physicochemical properties of the drug and physiological factors at the absorption site. Absorption can occur through various routes, including oral, inhalation, transdermal, intramuscular, and subcutaneous.
- Distribution: After absorption, medications are distributed throughout the body via the bloodstream. Distribution involves the transport of drugs from the systemic circulation to various tissues, organs, and target sites. Factors influencing drug distribution include blood flow to different tissues, drug lipophilicity, binding to plasma proteins and tissue permeability. Distribution can be affected by physiological factors like body composition, age and presence of diseases.
- Metabolism: Metabolism, also known as biotransformation, refers to the chemical alteration of medications in the body. The primary site of drug metabolism is the liver, although other organs like the intestine, lungs and kidneys can also contribute. Metabolism typically involves enzymatic reactions that convert drugs into metabolites, which may be inactive or active. The main purpose of drug metabolism is to facilitate elimination and inactivate potentially toxic substances. Cytochrome P450 enzymes play a crucial role in drug metabolism.
- Elimination: Elimination encompasses the processes by which medications and their metabolites are removed from the body. The main routes of elimination are renal (via urine) and hepatic (via bile and faeces). Other routes include pulmonary excretion (via exhaled air), sweat and breast milk. The rate of elimination is determined by factors such as drug clearance, renal function, hepatic function and drug interactions. The half-life of a drug represents the time required for its concentration in the body to decrease by half, and it is influenced by the drug’s elimination rate.
Speed of Efficacy
Speed of absorption and onset of action can vary depending on individual factors, medication properties, formulations and specific patient conditions.
- Intravenous (IV): The IV route is generally considered the fastest and most effective route of administration. Medications administered intravenously are directly delivered into the bloodstream, bypassing the absorption process. This allows for rapid and complete systemic distribution of the medication.
- Intraosseous (IO): IO administration is considered highly efficient and provides a rapid route of medication delivery, especially in emergency situations when IV access is difficult or delayed. Medications injected into the bone marrow space quickly reach systemic circulation.
- Inhalation: Inhalation is considered a fast and efficient route, particularly for medications targeting the respiratory system. When inhaled, medications directly reach the lung tissues, where they can be rapidly absorbed into the bloodstream. The high surface area and extensive blood supply in the lungs facilitate quick absorption.
- Intramuscular (IM): The IM route involves injecting medications into a muscle, typically the deltoid, gluteus maximus, or vastus lateralis. Absorption from muscle tissue is generally faster than oral administration. The rich blood supply to muscles allows for relatively rapid uptake of medications into the bloodstream.
- Subcutaneous (SC): Subcutaneous injections involve delivering medications into the layer of fat just beneath the skin. Absorption from subcutaneous tissue is slower compared to the previous routes but generally faster than oral administration. Medications are gradually absorbed into the capillaries and enter systemic circulation.
- Intranasal: Medications administered intranasally are absorbed through the nasal mucosa. This route allows for relatively rapid absorption due to the large surface area and rich blood supply in the nasal passages. However, the speed of absorption can vary depending on factors such as drug properties and formulation.
- Rectal: Medications administered rectally are absorbed through the rectal mucosa and enter systemic circulation. Absorption from the rectum is generally slower than the previous routes mentioned. The rectal route can be useful when oral administration is not feasible or when local effects are desired.
- Oral: The oral route, involving ingestion of medications, is considered slower than the aforementioned routes. Oral medications need to be dissolved, absorbed in the gastrointestinal tract, and pass through the liver before reaching systemic circulation. Factors such as gastrointestinal transit time, pH levels and first-pass metabolism can further affect the speed of absorption.
- Transdermal: Transdermal administration involves applying medications to the skin, from where they are slowly absorbed into systemic circulation over an extended period. This route provides a controlled and sustained release of medication, but absorption can be relatively slow compared to other routes.
- Topical: Topical medications are applied to the skin or mucous membranes for localised effects. Absorption is generally slower than the previous routes mentioned, as the medications primarily act locally rather than entering systemic circulation in significant amounts.