A patient uses ephedrine nasal drops three times daily for congestion. After four days, the decongestant effect is markedly diminished despite correct use. The most likely explanation is:

• A Depletion of noradrenaline stores from repeated release by the indirectly acting sympathomimetic ✓
• B Induction of hepatic CYP enzymes accelerating ephedrine metabolism
• C Downregulation of vascular alpha-adrenoceptors caused by sustained receptor activation
• D Accumulation of ephedrine causing receptor desensitisation through toxicity

Explanation

Ephedrine is an indirectly acting sympathomimetic that displaces noradrenaline from storage vesicles. Repeated dosing exhausts the releasable transmitter pool, so successive doses produce progressively smaller effects: tachyphylaxis. Hepatic enzyme induction takes days to weeks and is not relevant to topical use. While true receptor downregulation occurs with some agonists over longer periods, the classic and fastest mechanism for ephedrine loss of effect is vesicular noradrenaline depletion, a standard examination point.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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Written and medically reviewed by the StethoPrep medical team.