How does reduced GFR affect pharmacokinetics of renally cleared drugs?

• A. Shortened half-life.
• B. Prolonged half-life; requires dose reduction or interval extension.
• C. Increased renal clearance.
• D. No change in pharmacokinetics.

Answer: (B)

When GFR is reduced, renal clearance of drugs that are cleared by the kidneys decreases. Elimination half-life depends on clearance (t1/2 = 0.693 × Vd / CL), so a drop in clearance lengthens the half-life. This means the drug stays in the body longer and can accumulate if the same dose and interval are used. To keep exposure similar to normal function and avoid toxicity, you reduce the dose or extend the dosing interval. For renally cleared drugs, this adjustment is essential because reduced GFR directly slows elimination.