Case 31: The Antidepressant That Slowed the Heart — CYP2D6 and Metoprolol

Presentation

A 58-year-old man with hypertension and prior myocardial infarction is on metoprolol succinate 100 mg daily, lisinopril, and atorvastatin. His blood pressure has been at goal for two years and his resting heart rate has run in the mid-60s.

Six weeks ago his primary care physician started paroxetine 20 mg daily for major depressive disorder. He now presents with fatigue, exertional lightheadedness, and one near-syncopal episode while gardening.

Vitals and ECG

BP 104/58 · HR 42, regular · afebrile. ECG shows sinus bradycardia with Mobitz type I second-degree AV block. Potassium 4.1, creatinine 1.1 (baseline 1.0), TSH normal, troponin negative. He has not missed or doubled any doses.

Questions

1

His metoprolol dose has not changed in two years. What is the most likely explanation for the bradycardia and AV block?

A) Progression of underlying conduction system disease

B) Paroxetine inhibition of CYP2D6, raising metoprolol exposure

C) Silent reinfarction involving the AV node

D) Depression-related vagal hypertonia

Correct Answer: B
Learning Point: Metoprolol is a CYP2D6 substrate and paroxetine is a potent CYP2D6 inhibitor. Experimental data show paroxetine raises metoprolol AUC three- to five-fold with significant reductions in systolic blood pressure and heart rate; bradycardia and AV block are documented in case reports. The patient is effectively taking 300–500 mg of metoprolol. Answers A and C are possible but far less likely given the tight temporal link to a new drug and an otherwise unremarkable workup — and neither explains the simultaneous fall in blood pressure. D is not a real mechanism at this magnitude.
📚 Reference: Clinical Mastery: Psychotropic and ADHD Medications in Hypertension and CKD — Section 6.2

2

Which single medication change best resolves the interaction while still treating both conditions?

A) Halve the metoprolol dose and continue paroxetine

B) Stop the beta-blocker entirely

C) Switch paroxetine to sertraline or escitalopram

D) Add a pacemaker referral and change nothing

Correct Answer: C
Learning Point: Sertraline and escitalopram are weak CYP2D6 inhibitors, so substituting either removes the interaction and preserves both the antidepressant indication and post-MI beta-blockade. This is the single highest-yield substitution in psychotropic–antihypertensive prescribing. A leaves an unpredictable interaction in place and re-creates the problem at any future dose change. B abandons guideline-directed post-MI therapy for a reversible drug effect. D treats an iatrogenic, fully reversible problem with a permanent device.
📚 Reference: Clinical Mastery: Psychotropic and ADHD Medications in Hypertension and CKD — Section 6.2

3

If the patient had strongly preferred to stay on paroxetine, which alternative beta-blocker would avoid this interaction?

A) Carvedilol

B) Nebivolol

C) Propranolol

D) Atenolol or bisoprolol

Correct Answer: D
Learning Point: Atenolol and bisoprolol are not CYP2D6-dependent, so paroxetine does not meaningfully alter their exposure. Carvedilol, nebivolol, propranolol, timolol, and metoprolol are all CYP2D6 substrates and share the same vulnerability — a point commonly missed, since clinicians often switch within the class and unknowingly keep the interaction.
📚 Reference: Lecture: Psychiatric & ADHD Medications and Blood Pressure

4

Which OTHER commonly prescribed psychotropics carry this same CYP2D6 risk with beta-blockers?

A) Fluoxetine, bupropion, and duloxetine

B) Sertraline and mirtazapine

C) Buspirone and trazodone

D) Citalopram and escitalopram

Correct Answer: A
Learning Point: Fluoxetine, bupropion, and duloxetine all inhibit CYP2D6. Bupropion is the most frequently overlooked because it often arrives as a smoking-cessation drug prescribed by someone who is not managing the cardiac regimen. Buspirone's problem is CYP3A4 (diltiazem/verapamil), not CYP2D6.
📚 Reference: Student Handout — Section 5

Take-Home

The reasoning pattern worth keeping

When a stable patient destabilizes on an unchanged dose, the answer is usually a new drug altering the old drug's metabolism — not progression of disease. Check the medication list by date. If the beta-blocker is a CYP2D6 substrate and the new drug is paroxetine, fluoxetine, bupropion, or duloxetine, you have your mechanism.

Andrew Bland, MD, FACP, FAAP

Medical Associates Department of Nephrology · University of Illinois College of Medicine at Peoria · University of Dubuque PA & DPT Programs · Butler College of Osteopathic Medicine

Interactive teaching case · Psychotropic & ADHD Medications and Blood Pressure