Drug interactions
Phenytoin + Theophylline
Severity: Moderate
Phenytoin and Theophylline: moderate interaction.
Last updated Sources: US FDA label
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
The combination lowers theophylline levels, risking loss of bronchodilator control. Theophylline can also reduce phenytoin concentrations, risking seizure breakthrough.
What this interaction means in plain words
Phenytoin and Theophylline can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Phenytoin belongs to Central Nervous System; Theophylline belongs to Respiratory.
Why it happens
Phenytoin induces hepatic CYP enzymes (including CYP1A2) increasing theophylline clearance, while theophylline may reduce phenytoin absorption or increase its clearance.
What to do
Monitor serum theophylline and phenytoin concentrations and clinical response. Increase theophylline dose if needed and readjust when phenytoin is stopped.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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References
- Theophylline (Anhydrous) - FDA drug label (DailyMed) — openFDA / DailyMed
Other interactions of Theophylline
- Riociguat + Theophylline Contraindicated
- Adenosine + Theophylline Major
- Apalutamide + Theophylline Major
- Bupropion + Theophylline Major
- Carvedilol + Theophylline Major
- Cimetidine + Theophylline Major
Other interactions of Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Frequently asked questions
Can I take Phenytoin with Theophylline?
MediHelp rates the interaction as moderate. Phenytoin and Theophylline can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. The combination lowers theophylline levels, risking loss of bronchodilator control. Theophylline can also reduce phenytoin concentrations, risking seizure breakthrough.
How serious is the interaction between Phenytoin and Theophylline?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor serum theophylline and phenytoin concentrations and clinical response. Increase theophylline dose if needed and readjust when phenytoin is stopped.
What other medicines interact with Theophylline?
MediHelp also lists interactions of Theophylline with, for example, Riociguat, Adenosine, Apalutamide. The full list is on the Theophylline page.
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