Pharmacology Flashcards

A patient on phenytoin for epilepsy is started on rifampin for tuberculosis. The patient's seizures increase. What pharmacokinetic interaction explains this?

Answer: Rifampin induces hepatic CYP450 enzymes, accelerating phenytoin metabolism

Rifampin is a potent CYP450 inducer that increases phenytoin metabolism, reducing its plasma concentration and causing therapeutic failure.

Alcohol metabolism follows zero-order kinetics at typical drinking concentrations. What does this mean clinically?

Answer: A constant amount of alcohol is eliminated per unit time, regardless of concentration

Zero-order kinetics means a fixed amount (not fraction) is metabolized per unit time because the metabolizing enzyme (alcohol dehydrogenase) is saturated, making elimination concentration-independent.

A drug has a therapeutic index (TI) of 2. How does this TI compare to morphine (TI ~70) in terms of clinical management?

Answer: The drug with TI of 2 has a narrower safety margin requiring closer dose titration and monitoring

A low therapeutic index (TI = TD50/ED50) means the toxic dose is only twice the effective dose, requiring precise dosing to avoid toxicity.

A drug with high first-pass metabolism has very low oral bioavailability. Which route of administration would bypass first-pass hepatic metabolism?

Answer: Sublingual

Sublingual administration delivers drug directly into the systemic circulation via the sublingual veins, completely bypassing the portal circulation and hepatic first-pass metabolism.

A patient requires a loading dose of a drug to rapidly achieve therapeutic plasma levels. Which pharmacokinetic parameter primarily determines the loading dose?

Answer: Volume of distribution (Vd)

Loading dose = (Target concentration × Vd) / F; a large Vd means the drug distributes widely into tissues and requires a larger loading dose to achieve the desired plasma concentration.

A patient receiving a β2-agonist inhaler for asthma notices that the same dose produces less bronchodilation after 2 weeks of continuous use. This phenomenon is best described as:

Answer: Tolerance due to receptor desensitization and downregulation

Prolonged agonist exposure causes receptor phosphorylation, uncoupling from G-proteins, and downregulation of receptor number, reducing drug response over time.

Which statement best describes the pharmacodynamic concept of competitive antagonism?

Answer: The antagonist shifts the agonist dose-response curve to the right without reducing the maximum effect

Competitive antagonists compete reversibly for the same receptor site; increasing agonist concentration can overcome the blockade, shifting the dose-response curve rightward with preserved Emax.