Phenobarbital and Nirmatrelvir/Ritonavir Interaction
Drug interaction information between Phenobarbital and Nirmatrelvir/Ritonavir.
Phenobarbital and Nirmatrelvir/Ritonavir have a documented major interaction in FDA labeling.
FDA drug labeling documents a major-severity interaction between Phenobarbital and Nirmatrelvir/Ritonavir. This page displays the available source-record category and label text. The category is not an individual risk estimate or medication-combination direction.
How They Interact
Phenobarbital speeds up how fast your body breaks down the antiviral, which lowers the amount of medicine in your blood. This can stop the treatment from working and might lead to drug resistance.
FDA Label Information
Anticonvulsants carbamazepine , phenobarbital, primidone, phenytoin ↓ nirmatrelvir/ritonavir Co-administration contraindicated due to potential loss of virologic response and possible resistance [see Contraindications (4) ] .
Phenobarbital Also Interacts With
- Ranolazine major
- Voriconazole major
- Canagliflozin moderate
- Cenobamate moderate
- Clarithromycin moderate
Nirmatrelvir/Ritonavir Also Interacts With
- Simvastatin major
- Lovastatin major
- Sildenafil major
- Carbamazepine major
- Rifampin major
Frequently Asked Questions
What SPL severity band is on file for Phenobarbital + Nirmatrelvir/Ritonavir?
Compiled FDA labeling labels this pair major. That is a source-record field, not an individual medication-combination decision.
Which drug in this pair has the larger labeled-interaction portfolio?
Nirmatrelvir/Ritonavir has 86 labeled pairs on file vs Phenobarbital's 59. In the major band specifically: Phenobarbital has 3 (also Ranolazine, Voriconazole); Nirmatrelvir/Ritonavir has 23 (also Simvastatin, Lovastatin, Sildenafil, Carbamazepine).
What mechanism field is on file for Phenobarbital and Nirmatrelvir/Ritonavir?
Phenobarbital speeds up how fast your body breaks down the antiviral, which lowers the amount of medicine in your blood. This can stop the treatment from working and might lead to drug resistance.
Nirmatrelvir/Ritonavir carries more major-labeled pairs than Phenobarbital
SPL source-record counts only. This page's band is major · classes on file: Phenobarbital (Anticonvulsant (Barbiturate)); Nirmatrelvir/Ritonavir (Antiviral (Protease Inhibitor Combination)).
Nirmatrelvir/Ritonavir has 23 major-labeled pairs vs Phenobarbital's 3.
3 major · 13 moderate · 43 minor · 59 total
- Other major peers for Phenobarbital
- Ranolazine
- Voriconazole
23 major · 30 moderate · 33 minor · 86 total
- Other major peers for Nirmatrelvir/Ritonavir
- Simvastatin
- Lovastatin
- Sildenafil
- Carbamazepine
Sources: FDA Drug Labels (SPL) via openFDA (2026). This is for informational purposes only and is not medical advice. This registry does not provide individual medication-combination directions.
Read our methodology - how this data is sourced, computed, and verified.