Which medication may require a dose adjustment if the patient starts smoking?

Which medication may require a dose adjustment if the patient starts smoking?

• A. Amlodipine
• B. Levothyroxine
• C. Theophylline
• D. Fexofenadine

Answer: (C)

Smoking can increase the activity of certain liver enzymes, especially CYP1A2, which speeds up the metabolism of some drugs. Theophylline is largely cleared by CYP1A2, so when a patient starts smoking, theophylline is cleared more quickly and blood levels can drop. Because theophylline has a narrow therapeutic index, this faster clearance often means the dose or dosing frequency needs to be increased to maintain an effective level. If the patient later stops smoking, clearance falls again, raising the risk of toxicity, so the dose may need to be reduced. The other medications listed aren’t typically affected in dosing merely by starting to smoke. Amlodipine, levothyroxine, and fexofenadine don’t rely on CYP1A2 induction to the same clinically significant extent, and their dosing isn’t routinely adjusted just because a patient begins smoking.

Smoking can increase the activity of certain liver enzymes, especially CYP1A2, which speeds up the metabolism of some drugs. Theophylline is largely cleared by CYP1A2, so when a patient starts smoking, theophylline is cleared more quickly and blood levels can drop. Because theophylline has a narrow therapeutic index, this faster clearance often means the dose or dosing frequency needs to be increased to maintain an effective level. If the patient later stops smoking, clearance falls again, raising the risk of toxicity, so the dose may need to be reduced.

The other medications listed aren’t typically affected in dosing merely by starting to smoke. Amlodipine, levothyroxine, and fexofenadine don’t rely on CYP1A2 induction to the same clinically significant extent, and their dosing isn’t routinely adjusted just because a patient begins smoking.