Introduction to Drug Absorption
Drug absorption is the first stage of pharmacokinetics, describing the journey of a medication from its administration site into the body's systemic circulation [1.2.2]. For a drug to exert its intended physiological effect, it must first be absorbed. This process is influenced by the drug's physicochemical properties (like solubility and molecular size), its formulation (e.g., tablet, solution), and the route of administration [1.2.1, 1.2.6]. Unless a drug is administered intravenously (IV), it must cross several biological membranes to reach the bloodstream [1.2.1]. These membranes are lipid-based barriers that drugs traverse via mechanisms like passive diffusion, facilitated diffusion, or active transport [1.2.4, 1.2.6]. The route chosen is a critical decision based on the drug's characteristics, the desired speed of onset, and whether a local or systemic effect is needed.
Enteral Route: Through the Gastrointestinal Tract
The enteral route involves administering a drug through the gastrointestinal (GI) tract [1.3.4]. This is the most common, convenient, and economical method of drug administration [1.3.3].
Sub-routes of Enteral Administration
- Oral (PO): This involves swallowing medication, such as tablets, capsules, or liquids. Most absorption for orally administered drugs occurs in the small intestine due to its large surface area and high permeability [1.2.1]. However, this route subjects drugs to the harsh acidic environment of the stomach and enzymatic degradation [1.2.1].
- Sublingual and Buccal: Sublingual administration involves placing a drug under the tongue, while buccal administration is between the cheek and gum [1.7.3]. Both routes allow for rapid absorption directly into the bloodstream through the rich network of capillaries in the oral mucosa [1.7.1, 1.7.4]. A key advantage is bypassing the first-pass effect, where the liver metabolizes a significant portion of the drug, reducing its concentration before it reaches systemic circulation [1.7.1, 1.5.1].
- Rectal: Administered as suppositories, rectal delivery offers both local and systemic effects. This route is useful when oral administration is not possible. About 50% of a drug absorbed rectally bypasses the first-pass effect in the liver [1.3.5, 1.5.1].
Parenteral Route: Bypassing the GI Tract
Parenteral administration involves injecting a drug directly into the body, bypassing the enteral route [1.3.4]. This method is used for drugs that are poorly absorbed or unstable in the GI tract, or when a rapid onset of action is required [1.3.3]. Intravenous administration provides 100% bioavailability, as the drug enters the systemic circulation directly [1.3.3].
Sub-routes of Parenteral Administration
- Intravenous (IV): The drug is injected directly into a vein, leading to an immediate effect. It offers complete bioavailability but requires sterile technique and carries a higher risk of toxicity and infection [1.2.5].
- Intramuscular (IM): The drug is injected into a muscle. Absorption is typically faster than subcutaneous but depends heavily on blood flow to the injection site [1.2.1].
- Subcutaneous (SC or Sub-Q): The drug is injected into the fatty tissue just beneath the skin. This route provides slower, more sustained absorption compared to IM injections [1.2.5].
Other Major Routes of Absorption
- Inhalation: Drugs are delivered through the mouth or nose to be absorbed by the lungs. This route allows for rapid absorption due to the lungs' large surface area and extensive blood supply, making it effective for both local (e.g., asthma medication) and systemic effects [1.8.3, 1.8.5]. The particle size of the drug is a critical factor, as it determines how deeply the drug penetrates the pulmonary tree [1.8.3].
- Topical and Transdermal: These routes involve applying medication to the skin.
- Topical administration is intended for a local effect, with minimal penetration into the systemic circulation (e.g., creams for a skin rash) [1.6.6].
- Transdermal administration, often via a patch, is designed for the drug to penetrate the skin and enter the bloodstream for a systemic, controlled-release effect over a prolonged period [1.6.1, 1.6.3]. This route also avoids the first-pass effect [1.6.3].
| Route Comparison | Enteral (Oral) | Parenteral (IV) | Transdermal | Inhalation |
|---|---|---|---|---|
| Speed of Onset | Slow (variable) [1.2.1] | Very Rapid (immediate) [1.2.5] | Slow (sustained) [1.2.5] | Very Rapid [1.8.3] |
| Bioavailability | Variable, subject to first-pass effect [1.2.2] | 100% (complete) [1.3.3] | High (avoids first-pass) [1.6.3] | High (avoids first-pass) [1.8.5] |
| Common Use | General medications, chronic conditions [1.2.1] | Emergencies, hospitalized patients [1.3.3] | Sustained-release needs (e.g., pain, hormones) [1.6.5] | Respiratory conditions, anesthesia [1.8.3] |
| Patient Convenience | High [1.3.3] | Low (requires professional) [1.3.3] | High [1.6.5] | Moderate [1.2.2] |
Factors Affecting Drug Absorption
Several factors can influence how a drug is absorbed, regardless of the route:
- Drug Properties: Solubility, molecular size, and degree of ionization affect a drug's ability to cross cell membranes [1.4.2, 1.4.6]. Generally, lipid-soluble and smaller molecules are absorbed more easily [1.2.6].
- Physiological Factors: A person's age, the pH of the absorption site (like the stomach), blood flow to the area, and gastric emptying time can all impact absorption [1.4.3, 1.4.5]. For example, decreased blood flow in older adults can slow the absorption of drugs from transdermal patches or injections [1.2.5].
- Interactions: Food and other drugs can interfere with absorption. High-fiber foods, for instance, can bind to a drug and prevent it from being absorbed [1.4.3].
Conclusion
The choice of drug administration route is a crucial aspect of pharmacology that directly influences a medication's absorption, bioavailability, and ultimately its therapeutic success. From the convenience of the oral route to the immediacy of intravenous injection and the sustained delivery of transdermal patches, each method offers distinct advantages and disadvantages. A thorough understanding of what are the main routes of drug absorption, combined with knowledge of the first-pass effect and other influencing factors, allows healthcare professionals to optimize treatment for efficacy and patient safety. For more in-depth information, the Merck Manual provides authoritative details on pharmacokinetics. [1.2.1]
