Phenytoin and Carbidopa/Levodopa Interaction
Drug interaction information between Phenytoin and Carbidopa/Levodopa.
Phenytoin and Carbidopa/Levodopa have a documented moderate interaction in FDA labeling.
FDA drug labeling documents a moderate-severity interaction between Phenytoin and Carbidopa/Levodopa. 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
Phenytoin can interfere with how carbidopa and levodopa work in the body. This makes the Parkinson's medication less effective at controlling symptoms.
FDA Label Information
7 DRUG INTERACTIONS Drugs metabolized by COMT: use with caution ( 5.10 , 7.2 ) Anti-hypertensive agents: dose adjustment may be required ( 7.3 ) Tricyclic antidepressants: risk of hypertension and dyskinesia reported during concomitant use with carbidopa/levodopa ( 7.4 ) Dopamine D2 receptor antagonists, isoniazid, phenytoin, papaverine and iron salts: may reduce efficacy of carbidopa, levodopa and entacapone tablets ( 7.5 , 7.6 , 7.7 , 7.8 , 7.9 ) Drugs that interfere with biliary excretion, glucuronidation and intestinal beta-glucuronidase: dose adjustment of carbidopa, levodopa and...
Phenytoin Also Interacts With
- Nirmatrelvir/Ritonavir major
- Posaconazole major
- Ranolazine major
- Voriconazole major
- Apixaban moderate
Carbidopa/Levodopa Also Interacts With
- Warfarin moderate
- Entacapone moderate
- Methyldopa moderate
- Dobutamine moderate
- Dopamine moderate
Frequently Asked Questions
What SPL severity band is on file for Phenytoin + Carbidopa/Levodopa?
Compiled FDA labeling labels this pair moderate. That is a source-record field, not an individual medication-combination decision.
Which drug in this pair has the larger labeled-interaction portfolio?
Phenytoin has 147 labeled pairs on file vs Carbidopa/Levodopa's 13. In the moderate band specifically: Phenytoin has 23 (also Apixaban, Aprepitant, Canagliflozin, Clarithromycin); Carbidopa/Levodopa has 8 (also Warfarin, Entacapone, Methyldopa, Dobutamine).
What mechanism field is on file for Phenytoin and Carbidopa/Levodopa?
Phenytoin can interfere with how carbidopa and levodopa work in the body. This makes the Parkinson's medication less effective at controlling symptoms.
Phenytoin carries more major-labeled pairs than Carbidopa/Levodopa
SPL source-record counts only. This page's band is moderate · classes on file: Phenytoin (Anticonvulsant (Hydantoin)); Carbidopa/Levodopa (Dopamine Precursor Combination).
Phenytoin has 4 major-labeled pairs vs Carbidopa/Levodopa's 0. This page itself is in the moderate band, not the major band.
4 major · 23 moderate · 120 minor · 147 total
- Other moderate peers for Phenytoin
- Apixaban
- Aprepitant
- Canagliflozin
- Clarithromycin
0 major · 8 moderate · 5 minor · 13 total
- Other moderate peers for Carbidopa/Levodopa
- Warfarin
- Entacapone
- Methyldopa
- Dobutamine
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.