Hydrochlorothiazide and Flecainide: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Hydrochlorothiazide
Flecainide
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Increased risk of electrolyte imbalance and subsequent cardiotoxicity
Interaction Deep Dive
Concomitant use of flecainide and a thiazide diuretic is discouraged as this led to electrolyte imbalance and subsequent cardiotoxicity (QRS widening) in a 63-year-old hypertensive man1. However, if the benefits of using these 2 agents together outweigh the risks, closely monitor electrolytes and symptoms of arrhythmias, including syncope.
Why it happens (mechanism)
Unknown
Literature reports
1 report — tap to read
a) A 63-year-old man on long-term bendroflumethiazide experienced electrolyte imbalance and subsequent cardiotoxicity 2 months after starting flecainide. His medications included bendroflumethiazide 2.5 mg once daily and felodipine 2.5 mg once daily for hypertension. Within 2 months of starting flecainide 100 mg twice daily for prophylaxis of narrow complex tachycardia, he experienced progressive fatigue, muscle weakness, and eventually recurrent syncope. The syncopal episodes came without warning, were accompanied by loss of consciousness for seconds, and resolved promptly and spontaneously. On admission, he was hyponatremic (serum sodium of 118 mmol/mL) and hypokalemic (serum potassium of 2.7 mmol/mL), his ECG demonstrated sinus rhythm with marked QRS widening, and his blood pressure was 125/80 mmHg. Bendroflumethiazide and flecainide were discontinued, and the electrolyte imbalance as well as the ECG and syncope corrected completely 1.
Common questions
Can I take Hydrochlorothiazide and Flecainide together?
Increased risk of electrolyte imbalance and subsequent cardiotoxicity Always confirm with your pharmacist or prescriber before making any change.
How serious is the Hydrochlorothiazide and Flecainide interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Hydrochlorothiazide or Flecainide need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (1)
- Khavandi A & Walker PR: Flecainide cardiotoxicity precipitated by electrolyte imbalance. Caution with thiazide diuretics. Emerg Med J 2007; 24(5):e26-. PubMed
Keep reading about Hydrochlorothiazide
Keep reading about Flecainide
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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