Pharmacokinetics and Drug Action: Key Concepts Explained
Why Elderly May Experience Increased Sleepiness with Morphine
Elderly individuals, similar to children, often have reduced hepatic activity. This means that drugs like morphine take longer to be metabolized and eliminated from the body, potentially leading to increased drowsiness even after withdrawing the drug.
Drug Molecule Characteristics for Crossing Physiological Barriers
Drugs with smaller, non-ionized, and lipid-soluble molecules can easily cross physiological membranes via passive diffusion. Water-soluble substances of very small size can also penetrate cells through membrane pores.
Importance of Plasma Half-Life for Nurses
Understanding plasma half-life is crucial for nurses because it determines the frequency and timing of drug administration. In some cases, drugs need to be administered with infusion pumps, and knowing the half-life helps determine the appropriate dosage for the patient.
The First-Pass Effect Explained
When a drug enters the body, it can be degraded in various ways. A portion of the administered dose may never reach its intended target because it is inactivated or destroyed before reaching the target cells. This phenomenon is known as the “first-pass effect.” The most significant aspect is the first-pass metabolism, which affects substances administered orally.
Prodrugs: Definition and Use
Prodrugs are inactive substances that lack pharmacological activity upon administration. They require metabolism to be converted into an active form. Examples include enalapril and losartan.
Molecular Characteristics for Easy Elimination
Molecules that are readily water-soluble and exist in their ionized form within the body are more easily eliminated.
Narrow Therapeutic Range: Meaning and Risks
A narrow therapeutic range indicates a small difference between the effective dose and the toxic dose. This means that the drug can become toxic if administered in slightly higher doses or if it remains in the body for an extended period.
Therapeutic Index and Drug Safety
A drug with a higher therapeutic index is safer than one with a lower index. A larger therapeutic index indicates a greater safety margin. A value close to 1 is extremely dangerous. Conversely, a drug with a therapeutic index of 4 is considered very safe because it would require four times the effective dose to produce a toxic effect.
Enteric-Coated vs. Film-Coated Tablets
- Film-coated tablets: The coating protects the active ingredient from light and moisture and facilitates swallowing.
- Enteric-coated tablets: These tablets pass through the stomach without dissolving, releasing the active substance in the intestine. They should not be crushed.
Delayed-Release Tablets Explained
Delayed-release tablets initiate drug release in the gut, passing through the stomach intact. These oral forms are enteric-coated and resist stomach acid secretions, ultimately disintegrating in the small intestine. They are used to protect drugs that are altered by gastric juices or to protect the gastric mucosa from irritants. These drugs typically start acting 2-4 hours after administration.