Drug interactions
Fosphenytoin + Theophylline
Severity: Moderate
Fosphenytoin 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
Phenytoin, the active form of fosphenytoin, induces hepatic enzymes and can lower theophylline levels. Theophylline may also lower phenytoin levels. Both effects can cause loss of control of asthma or seizures.
What this interaction means in plain words
Fosphenytoin 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.
Fosphenytoin belongs to Central Nervous System; Theophylline belongs to Respiratory.
Why it happens
Phenytoin induces CYP1A2 and CYP3A4, increasing theophylline clearance; theophylline may reduce phenytoin concentrations.
What to do
Monitor both theophylline and phenytoin concentrations and adjust doses as needed.
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
- Fosphenytoin sodium - 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 Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir Contraindicated
Frequently asked questions
Can I take Fosphenytoin with Theophylline?
MediHelp rates the interaction as moderate. Fosphenytoin 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, the active form of fosphenytoin, induces hepatic enzymes and can lower theophylline levels. Theophylline may also lower phenytoin levels. Both effects can cause loss of control of asthma or seizures.
How serious is the interaction between Fosphenytoin 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 both theophylline and phenytoin concentrations and adjust doses as needed.
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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