The short answer
High-dose isolated daidzein increased theophylline levels in a human study. Have a soy isoflavone supplement reviewed before starting it.
What this answer covers
Soy isoflavones; isolated daidzein 400 mg/day research cannot represent all isoflavone mixtures or ordinary soy foods.
- Supplement or preparation
- Soy Isoflavones
- Medication ingredient
- theophylline
What the direct study showed
Healthy volunteers receiving daidzein for ten days had higher exposure to a single oral theophylline dose. The study also found a change in a marker of the liver enzyme involved in drug metabolism.
The supplement composition matters
The experiment used 200 mg daidzein twice daily. A mixed soy isoflavone capsule can have a different composition, and ordinary soy foods should not automatically be assigned the same effect.
Use a monitored plan
Keep theophylline dosing and formulation consistent with your prescription. Tell the prescriber about supplement starts, stops and dose changes. Separating products by a few hours has not been shown to prevent changes in drug clearance, and a study dose is not a recommended supplement dose.
Recognize possible toxicity
Contact the prescriber promptly for repeated nausea or vomiting, new insomnia or tremor after a product change. A seizure, fainting or a sustained rapid or irregular heartbeat needs urgent assessment. A blood theophylline measurement may be needed; symptoms alone cannot determine the level.
Evidence and practical considerations
Absorption & effectiveness
A direct study of high-dose daidzein supports a conditional soy-isoflavone concern. It found higher theophylline exposure with metabolic inhibition measures, not proof of increased gut absorption from all soy products.
- Exact scope
- Soy isoflavones; isolated daidzein 400 mg/day research cannot represent all isoflavone mixtures or ordinary soy foods. with Exact theophylline anhydrous extended-release tablets and oral solution. Exact RxNorm IN10438. Oral anhydrous theophylline formulations distinguished; intravenous and aminophylline study exposures are identified separately.
- Medication form and route
- oral
- Timing or duration
- Source-specific single-dose, repeated-dose and observational exposures distinguished.
What remains uncertain
- Short study in healthy adults using one isolated constituent; no dose-response safety threshold, toxicity rate or validated separation interval.
Factors that may matter: preparation and plant part; clinical indication; dose; other medicines; baseline alertness.
Read the supporting source summaries
Source 1: current product label
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: People prescribed theophylline.
Variable clearance, narrow therapeutic range and serum concentration monitoring; toxicity can include arrhythmias and seizures; label specifically warns about St John’s wort and ephedrine.
How this applies: Exact RxNorm IN10438. Oral anhydrous theophylline formulations distinguished; intravenous and aminophylline study exposures are identified separately.
No universal supplement dose, interval, event rate or untested formulation equivalence.
Source 2: current product label
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: People prescribed theophylline.
Variable clearance, narrow therapeutic range and serum concentration monitoring; toxicity can include arrhythmias and seizures; label specifically warns about St John’s wort and ephedrine.
How this applies: Exact RxNorm IN10438. Oral anhydrous theophylline formulations distinguished; intravenous and aminophylline study exposures are identified separately.
No universal supplement dose, interval, event rate or untested formulation equivalence.
Source 3: original human study
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Twenty healthy volunteers, placebo-controlled parallel study.
Theophylline AUC increased about one third and peak concentration about one quarter; CYP1A2 probe ratio decreased.
How this applies: Human constituent evidence, not a direct study of this whole botanical or variable isoflavone mixture. Only the named drug, preparation, route and measured endpoints apply.
High-dose isolated daidzein, not all soy isoflavone mixtures or foods; no clinical toxicity rate established.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Research summary published | |
| Article prepared | |
| Current exact available claims adjudicated using original captures and explicit identity and comparator limits. |
Article revision: ec80f83da1335037
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.