Fosfomycin and Amitriptyline: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jul 2, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Amitriptyline
Fosfomycin
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.
Both of these medicines can affect your heart's rhythm. Amitriptyline (Elavil) and the antibiotic fosfomycin can each slightly slow down the way your heart resets between beats. This is called QT prolongation. When you take both together, that effect can add up, which in rare cases could lead to an irregular heartbeat.
The good news is that this concern is based on theory, not on lots of real-world cases. Fosfomycin is also usually a short course. Please don't stop either medicine on your own. Talk with your pharmacist or doctor so they can check whether both are still the best choice for you and keep an eye on things.
Mechanism: additive QT interval prolongation. Both amitriptyline (a tricyclic antidepressant with known QT-prolonging potential) and fosfomycin carry QT-prolongation labeling; combined use may increase risk of ventricular arrhythmias including torsades de pointes. This is a pharmacodynamic (additive) interaction, not a PK-based one. Neither agent activation depends on the other.
- Direction: increased combined QT effect
- Evidence: theoretical
- Onset: unspecified
- Management: avoid coadministration where possible; if used together, monitor and correct electrolytes (K+, Mg2+), consider ECG monitoring, and assess other QT risk factors (bradycardia, other QT-prolonging drugs, hepatic/cardiac disease).
What happens
An increased risk of QT interval prolongation
Interaction Deep Dive
Fosfomycin should not be given together with other agents recognized for prolonging the QT interval, since doing so can heighten the potential for ventricular arrhythmias. Electrolytes should be monitored while a patient is receiving fosfomycin1.
Why it happens (mechanism)
Additive QT interval prolongation
How to manage this interaction
Here is how a care team typically handles this:
- Preferred approach: when possible, they may avoid using these two together, or choose an alternative antibiotic that does not affect the QT interval.
- If both are needed: your team may check and correct your blood electrolytes (like potassium and magnesium) during fosfomycin treatment, and may monitor your heart rhythm more closely.
Keep taking both exactly as prescribed unless told otherwise. Let your pharmacist or prescriber know if you take other QT-affecting drugs, or if you feel faint, dizzy, or notice a fluttering or racing heartbeat.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Fosfomycin and Amitriptyline together?
Amitriptyline and fosfomycin can each prolong the QT interval, and using them together may add up that effect, so tell your care team and let them decide whether to avoid the combination or monitor you closely. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Fosfomycin and Amitriptyline interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How is the Fosfomycin and Amitriptyline interaction managed?
Here is how a care team typically handles this: Preferred approach: when possible, they may avoid using these two together, or choose an alternative antibiotic that does not affect the QT interval. If both are needed: your team may check and correct your blood electrolytes (like potassium and magnesium) during fosfomycin treatment, and may monitor your heart rhythm more closely. Keep taking both e… Management is individual — always follow your own care team's guidance.
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 Fosfomycin or Amitriptyline 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)
- Product Information: CONTEPO intravenous injection, fosfomycin intravenous injection. Meitheal Pharmaceuticals Inc (per FDA), Chicago, IL, 2025. DailyMed
Keep reading about Amitriptyline
Keep reading about Fosfomycin
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.
Check another combination
Our instant two-drug interaction checker is almost here.
Still have questions about this combination?
Every question gets a real answer from a licensed pharmacist — free, and usually within a day.
Ask the pharmacist