Clozapine and Letermovir Injection: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Clozapine
Letermovir Injection
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 cloZAPine exposure
Interaction Deep Dive
Concomitant use of cloZAPine (CYP3A4 substrate) and a CYP3A4 inhibitor may lead to increased cloZAPine levels and increased risk for adverse reactions. If used concomitantly, monitor for adverse reactions and consider cloZAPine dose reduction as necessary. If this drug is discontinued, monitor for lack of cloZAPine effectiveness and consider increasing the dose1.
Why it happens (mechanism)
Inhibition of CYP3A4-mediated metabolism of cloZAPine
Literature reports
2 reports — tap to read
a) Concomitant administration of nefazodone may result in decreased clearance resulting in elevated plasma concentrations of cloZAPine and norclozapine. A 40-year-old man with a history of schizophrenia was successfully treated with cloZAPine and risperidone for several years. After experiencing persistent negative symptoms, nefazodone was initiated at 200 mg/day for 7 days and then increased to 300 mg/day. Seven days later, the patient reported increased anxiety and dizziness. Physical exam revealed mild hypotension. An increase in plasma concentrations and decrease in clearance of both cloZAPine and norclozapine was documented. Nefazodone dose was reduced to 200 mg/day and, within 1 week, the patient's symptoms and hypotension resolved. Nefazodone may cause a modest, dose-dependent reduction in the clearance of both cloZAPine and norclozapine, with resultant increases in serum concentrations. The author suggests that this effect may be due to nefazodone inhibition of the cytochrome P450 3A4 isoenzyme. Caution is suggested when prescribing nefazodone concomitantly with cloZAPine 2.
b) Six patients receiving a stable dose of cloZAPine for at least 2 weeks were selected to begin nefazodone therapy at 100 mg twice daily for 1 week and then 200 mg daily for 2 more weeks. The overall changes in cloZAPine pharmacokinetics were minimal when nefazodone was coadministered. CloZAPine concentrations increased by an average of 19 mcg/L (4% of baseline) and norclozapine concentrations increased by 46 mcg/L (16% of baseline). Cytochrome P450 3A4 (CYP3A4) has been postulated to play a significant role in the metabolism of cloZAPine. Nefazodone is an inhibitor of CYP3A4. Because this study failed to show a significant interaction between these 2 drugs, CYP3A4 may play only an insignificant role in the metabolism of cloZAPine, or alternative routes of metabolism may be activated when CYP3A4 is inhibited 3.
Common questions
Can I take Clozapine and Letermovir Injection together?
Increased cloZAPine exposure Always confirm with your pharmacist or prescriber before making any change.
How serious is the Clozapine and Letermovir Injection 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 strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
Questions for your pharmacist
- Does my dose of Clozapine or Letermovir Injection 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 (3)
- Product Information: CLOZARIL(R) oral tablets, clozapine oral tablets. HLS Therapeutics USA Inc (per FDA), Rosemont, PA, 2020. DailyMed
- Khan A & Preskorn S: Increase in plasma levels of clozapine and norclozapine after administration of nefazodone. J Clin Psychiatry 2001; 62(5):375-376. PubMed
- Taylor D, Bodani M, Hubbeling A, et al: The effect of nefazodone on clozapine plasma concentrations. Int Clin Psychopharmacol 1999; 14:185-187. DOI
Keep reading about Clozapine
Keep reading about Letermovir Injection
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