Amitriptyline and Fosamprenavir: 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
Fosamprenavir
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's going on: Fosamprenavir (Lexiva) is an HIV medicine that can slow down one of the liver pathways your body uses to break down amitriptyline (Elavil). When that happens, amitriptyline can build up in your blood a bit more than expected.
If levels climb, you might notice more of amitriptyline's usual side effects, like dry mouth, constipation, drowsiness, confusion, or, less commonly, changes in your heart rhythm. The good news is this is a theoretical concern, meaning it's based on how the drugs work rather than lots of real-world reports. Your care team can manage it easily by watching how you feel and adjusting things if needed. Please don't stop either medicine on your own, just bring it up with your pharmacist or doctor.
Mechanism: Fosamprenavir is a prodrug of amprenavir, a CYP3A4 inhibitor (and substrate). Amitriptyline is partially metabolized via CYP3A4, so inhibition may reduce its clearance and increase serum concentrations.
Direction/effect: Increased amitriptyline exposure, raising risk of anticholinergic effects, sedation, confusion, and cardiac arrhythmias (QT/conduction changes).
- Evidence: Theoretical; magnitude not quantified.
- Onset: Unspecified.
- Management: If coadministration is unavoidable, monitor amitriptyline plasma concentrations and watch for TCA toxicity. Consider baseline/periodic ECG. Dose may need individualized adjustment by the care team.
What happens
Increased tricyclic agent serum concentrations and potential toxicity (anticholinergic effects, sedation, confusion, cardiac arrhythmias)
Interaction Deep Dive
When fosamprenavir is given together with a tricyclic antidepressant, serum levels of the tricyclic antidepressant may rise, which introduces a possible risk of arrhythmias or other serious adverse effects. As a prodrug of amprenavir, fosamprenavir yields a compound that both inhibits the CYP3A4 isoenzyme and serves as a CYP3A4 substrate. Metabolism of tricyclic agents may rely in part on the CYP3A4 pathway. In patients who are also taking fosamprenavir, plasma concentrations of the tricyclic agent warrant close monitoring1.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated tricyclic agent metabolism
How to manage this interaction
Keep taking both medicines as prescribed unless your prescriber tells you otherwise. This is a manageable combination.
- Your care team may check amitriptyline blood levels and can tailor your dose if needed.
- They may monitor you more closely for signs of too much amitriptyline: dry mouth, constipation, heavy drowsiness, confusion, or a racing/irregular heartbeat.
- An ECG (heart tracing) may be used to keep an eye on your heart rhythm.
Tell your pharmacist or doctor if you feel unusually sleepy, foggy, dizzy, or notice your heart pounding or skipping.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Amitriptyline and Fosamprenavir together?
Fosamprenavir can raise amitriptyline levels by slowing its CYP3A4 breakdown, so your care team may monitor levels, watch for side effects, and adjust the dose if needed. Keep taking both as prescribed and report any drowsiness, confusion, or heart-rhythm changes. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Amitriptyline and Fosamprenavir interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How is the Amitriptyline and Fosamprenavir interaction managed?
Keep taking both medicines as prescribed unless your prescriber tells you otherwise. This is a manageable combination. Your care team may check amitriptyline blood levels and can tailor your dose if needed. They may monitor you more closely for signs of too much amitriptyline: dry mouth, constipation, heavy drowsiness, confusion, or a racing/irregular heartbeat. An ECG (heart tracing) may be used… 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 Amitriptyline or Fosamprenavir 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 anything you'd monitor while I'm on both?
References (1)
- Product Information: LEXIVA(R) oral solution, oral tablets, fosamprenavir calcium oral solution, oral tablets. GlaxoSmithKline, Research Triangle Park, NC, 2009. DailyMed
Keep reading about Amitriptyline
Keep reading about Fosamprenavir
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