Pharmacology
Summary
Drug administration can be classified into two primary categories: local and systemic. Local administration targets a specific area for its effect, such as topical creams applied to the skin or certain oral medications like sulfasalazine aimed at the intestinal lumen. On the other hand, drugs needing systemic circulation for delivery to their action site are administered systemically. Two invasive systemic routes are intravenous (IV) and intraarterial (IA) administration. Both provide a rapid onset and are ideal for drugs poorly absorbed through other channels like the GI tract. However, they require professional expertise and aren't suitable for self-administration. These methods ensure a bioavailability (F) of 1, bypassing first-pass metabolism. Two other systemic routes include intramuscular (IM), where medications are injected deep into specific muscles, and subcutaneous (SC), which involves injection just below the dermis. Both routes are alternatives to the GI tract, especially useful for drugs sensitive to stomach acids or those too large or polar to cross intestinal membranes.
For rapid response, methods such as buccal and sublingual administration are beneficial. They bypass the hepatic circulation and avoid first-pass metabolism, thereby increasing bioavailability. Drug permeation across the mouth's epithelium mainly occurs through paracellular diffusion, influenced by factors like charge, size, and degree of keratinization. Another alternative is rectal administration, suitable for individuals unable to take oral drugs. Drugs given rectally typically have a faster onset than those given orally but a shorter duration. Transdermal drug delivery, using patches, creams, or gels, is reserved for small, lipophilic, low-dose drugs. However, caution is advised with patches; heat sources can increase drug delivery rates, leading to potential overdose risks. Following patch removal, the residual drug can pose accidental exposure risks, demanding careful disposal. Other routes include intranasal administration, which offers rapid delivery but is limited by dosage constraints and potential irritation, and pulmonary administration, which not commonly used for systemic delivery due to safety concerns.
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FAQs
IV and IA drug administration provide a fast onset of action and full bioavailability (F) of 1 (100%). They are particularly useful for drugs that are poorly absorbed through other routes, such as the GI tract. IV and IA administration bypasses first-pass metabolism, which can increase bioavailability. However, these are invasive methods of administration that require skilled administration. Patients cannot self-administer drugs via these routes.
IM and SC methods are typically used when avoidance of the GI tract is necessary. They prevent exposure to the acidic environment of the stomach, making them suitable for drugs such as protein-based medications that might be destroyed by stomach acid. These methods also allow for the administration of drugs that are either too large or too polar (hydrophilic) to permeate intestinal membranes. Both IM and SC administration routes bypass the liver's first-pass metabolism, further enhancing the drug's bioavailability.
Buccal and sublingual routes offer a fast route for drug absorption, making them advantageous in situations where a rapid response is required. They also bypass hepatic circulation, thereby avoiding first-pass metabolism by the liver and increasing bioavailability. However, keratinized areas of the mouth can impede drug permeation. The success of these administration methods largely depends on the drug permeating via paracellular diffusion, determined by factors such as charge, size, and degree of keratinization.
The rectal administration is particularly useful for people who are unable to take drugs orally. Drugs administered rectally generally have a higher bioavailability and faster onset of action than those administered orally. The epithelium of the rectum is lipoidal, meaning lipid-soluble and nonionized drugs will permeate most readily. However, drugs inserted too high into the rectum may undergo extensive first-pass metabolism due to absorption via the superior rectal vein, which drains into the portal system
Transdermal administration, which includes patches, creams, and gels, is limited to small, lipophilic, low-dose drugs, permitting permeation through the stratum corneum. Patient counseling is essential to ensure correct usage - heat, for instance, can increase the rate of drug delivery and risk an overdose. Even after a patch is removed, the drug may remain present in the patch's matrix, posing a risk of accidental exposure. Thus, correct disposal is imperative; the patch should be folded in on itself and placed out of the reach of children.
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