Mr. John Smith, a 58-year-old male, is prescribed citalopram 20 mg daily for depression. He is also taking ranitidine 150 mg twice daily for dyspepsia. He approaches the pharmacy counter complaining of palpitations and dizziness. On review of his medication profile, you note he also started clarithromycin 500 mg twice daily three days ago for a respiratory tract infection. What is the most likely cause of Mr. Smith's symptoms?

Question: Mr. John Smith, a 58-year-old male, is prescribed citalopram 20 mg daily for depression. He is also taking ranitidine 150 mg twice daily for dyspepsia. He approaches the pharmacy counter complaining of palpitations and dizziness. On review of his medication profile, you note he also started clarithromycin 500 mg twice daily three days ago for a respiratory tract infection. What is the most likely cause of Mr. Smith's symptoms?

Answer options:

• Serotonin Syndrome due to the combination of citalopram and clarithromycin. ✅ Increased citalopram levels leading to QT prolongation.
• Dehydration from the clarithromycin-induced diarrhea.
• Hypoglycemia exacerbated by ranitidine.

Correct answer: Increased citalopram levels leading to QT prolongation.

Explanation: Clarithromycin is a potent CYP3A4 inhibitor. Citalopram is metabolized by CYP3A4 and CYP2C19. Inhibition of its metabolism by clarithromycin can lead to increased citalopram concentrations, elevating the risk of QT prolongation, which can manifest as palpitations and dizziness.