Drug & supplement interaction

Caffeine + Fosphenytoin

Fosphenytoin brand name: Cerebyx.

Moderate Interaction
Medically reviewed by · Updated September 2026
The bottom line Moderate

Caffeine has a moderate interaction with Fosphenytoin. Theoretically, caffeine might reduce the effects of phenytoin and increase the risk for convulsions.

Read the full interaction report
SeverityModerate
LikelihoodPossible
EvidenceD
What do Severity, Likelihood & Evidence mean?
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Interaction report

Caffeine and Fosphenytoin

Here is the interaction we found between these two, with the evidence behind it.

Moderate

PHENYTOIN (Dilantin)

Theoretically, caffeine might reduce the effects of phenytoin and increase the risk for convulsions.
Animal research suggests that caffeine can decrease the anticonvulsant activity of phenytoin. The effect does not seem to be related to the seizure threshold-lowering effects of caffeine. However, the exact mechanism of this interaction is unclear.
Severity Moderate Likelihood Possible Evidence D

Interaction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the short version: fosphenytoin is the injectable form of phenytoin, and your body quickly converts it into active phenytoin to calm down the abnormal electrical activity that triggers seizures. The concern on file is that caffeine may chip away at how well phenytoin controls seizures, which could theoretically raise the chance of a breakthrough convulsion.

I want to be honest about the strength of this, though. It comes from animal research only, and even the researchers weren't sure exactly how it happens. There's no solid proof it plays out the same way in people. Since fosphenytoin is usually given in a hospital for serious seizures and phenytoin has a narrow safe range, this is worth mentioning to your doctor or pharmacist, but it isn't a reason to panic over a cup of coffee.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Jankiewicz, K., Chroscinska-Krawczyk, M., Blaszczyk, B., and Czuczwar, S. J. [Caffeine and antiepileptic drugs: experimental and clinical data]. Przegl.Lek. 2007;64(11):965-967.
  2. Chroscinska-Krawczyk, M., Jargiello-Baszak, M., Walek, M., Tylus, B., and Czuczwar, S. J. Caffeine and the anticonvulsant potency of antiepileptic drugs: experimental and clinical data. Pharmacol.Rep. 2011;63(1):12-18. PubMed

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
  • ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
  • PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
  • UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
Evidence the quality of the studies behind the rating (graded A–D)
  • AHigh-quality meta-analysis (quantitative systematic review).
  • BCase-control study.
  • CConsensus.
  • DAnecdotal evidence.

These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.

Keep it in perspective

Caffeine Also Interacts With Other Medications

Caffeine has 8 major, 482 moderate, 164 minor known drug interactions in our database — the Fosphenytoin one above is one of them. If you take anything else, check it against the full list below.

Major 8 Moderate 482 Minor 164 No known interaction 2,176 (incl. Fosphenytoin)
Taking anything else?

Check Another of Your Medications Against Caffeine

Covers the 2,830 medications we’ve checked against Caffeine. Pick one to open its full report.

From our Q&A

Questions About Caffeine or Fosphenytoin

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Caffeine or Fosphenytoin, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Fosphenytoin

This page covers Caffeine only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Fosphenytoin page lists 205 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: 1,4-butanediol (major), American Hellebore (major), Arsenic (major), Beer (major) and Cesium (major).

By severity: 14 major · 172 moderate · 19 minor.

From our pharmacist’s overview on the Fosphenytoin page

“St. John's wort is the entry I would flag first with fosphenytoin (Cerebyx).”

Reviewed by 2 · Jul 18, 2026

Open the Fosphenytoin interaction page

One more smart check

Taking Caffeine as Part of a Dietary Supplement Product? Check It for Interactions

We hold 6,775 real supplement products containing Caffeine, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Caffeine. Your product may carry other ingredients that interact with Fosphenytoin too, on top of Caffeine — and those would not show on this page. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Caffeine and Fosphenytoin Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Caffeine
Top 5%

#24 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Fosphenytoin
Steady interest

#2,138 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Supplement

About Caffeine

What is Caffeine?

  • Mental alertness and reducing fatigue
  • Improving athletic performance
  • Headache and migraine relief
  • Boosting focus and concentration
  • Weight loss aid

Caffeine is a natural stimulant found in coffee, tea, and many other plants and products. In moderate amounts it can boost alertness and reduce tiredness for most healthy adults, but too much can cause jitteriness, a racing heart, and sleep problems. Pregnant women and people with heart conditions or anxiety should be especially careful and check with a doctor or pharmacist.

Read the full Caffeine monograph & all interactions
Medication

About Fosphenytoin Injection

What Is Fosphenytoin Injection Used For?

Brand name: Cerebyx

Fosphenytoin injection is used to treat generalized tonic-clonic status epilepticus (a prolonged, life-threatening seizure emergency) and to prevent or treat seizures that occur during or after brain surgery. It is given by intravenous (IV) or intramuscular (IM) injection in a hospital or clinical setting.

It can also be used short-term as a substitute for oral phenytoin when a patient cannot take medicine by mouth. This use is temporary — as soon as oral phenytoin is possible again, it should replace the injection.

See full Fosphenytoin Injection drug information
Keep checking

Other Drugs & Caffeine

Browse every other medication we’ve checked against Caffeine — both the ones with a known interaction and the ones without. Select any drug for the full detail.

8Major
482Moderate
164Minor
2,175No known interaction
8 Major 482 Moderate 164 Minor 2,175 No interaction
Major

8 interactions

Moderate

482 interactions

Showing 60 of 482 moderate interactions — see all 482 on the Caffeine page.

Minor

164 interactions

Showing 60 of 164 minor interactions — see all 164 on the Caffeine page.

No interaction 2,175 drugs checked — show the list

We also checked Caffeine against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,175 medications checked with no known interaction — see the full Caffeine list.

See the full Caffeine interactions page
See every supplement checked against Fosphenytoin, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

Keep exploring
Caffeine monograph › Uses, safety, and every drug interaction in one place Fosphenytoin supplement interactions › Every supplement checked against Fosphenytoin, ranked by severity Fosphenytoin Injection drug information › The full plain-English guide to this medication Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking Caffeine and Fosphenytoin Together: Common Questions

Can you take Caffeine with Fosphenytoin?
There is a moderate interaction, so it may be acceptable with monitoring or timing changes — but check with your pharmacist first. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Caffeine and Fosphenytoin interaction?
We rate it Moderate. This combination may require monitoring, a dose adjustment, or separating the times you take each one. Consult your pharmacist before using them together.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Still Have a Question About Fosphenytoin and Caffeine?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.