Drug & supplement interaction

Theanine + Phenobarbital

Phenobarbital brand names: Phenobarbital, Solfoton, Luminal.

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

Theanine has a minor interaction with Phenobarbital. Theoretically, theanine might have additive sedative effects when used in conjunction with CNS depressants.

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

Theanine and Phenobarbital

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

Minor

CNS DEPRESSANTS

Why this applies to Phenobarbital

Phenobarbital falls under CNS DEPRESSANTS — the group Theanine interacts with.

Theoretically, theanine might have additive sedative effects when used in conjunction with CNS depressants. However, it is unclear if this concern is clinically relevant.
Theoretically, theanine may compete with glutamate and/or increase plasma gamma-aminobutyric acid (GABA) levels, which could cause CNS depression. In one clinical study, some subjects taking oral theanine reported drowsiness.
Severity Minor Likelihood Unlikely Evidence D

Interaction has been demonstrated in animal or in lab research but has been shown not to occur in humans..

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

Here's the simple version. Phenobarbital is a barbiturate that works by calming activity in your brain, so it already makes many people feel drowsy or sleepy. Theanine, the calming amino acid from tea, is thought to nudge brain chemicals like GABA in a similar relaxing direction. Put them together and the worry is that the sedation could stack up a little, leaving you more tired, foggy, or slower to react than usual.

That said, this is only a theoretical concern based on how each works, not something proven in real patients. It's rated minor and unlikely. Still, since phenobarbital is a strong medication, I'd run it by your pharmacist or doctor before adding theanine, and go easy on activities like driving until you know how you feel.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Hidese S, Ota M, Wakabayashi C, et al. Effects of chronic l-theanine administration in patients with major depressive disorder: an open-label study. Acta Neuropsychiatr 2017;29(2):72-9.
  2. Tsuchiya T, Honda H, Oikawa M, et al. Oral administration of the amino acids cystine and theanine attenuates the adverse events of S-1 adjuvant chemotherapy in gastrointestinal cancer patients. Int J Clin Oncol 2016;21(6):1085-90. 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

Theanine Also Interacts With Other Medications

Theanine has 172 moderate, 392 minor known drug interactions in our database — the Phenobarbital one above is one of them. If you take anything else, check it against the full list below.

Moderate 172 Minor 392 No known interaction 2,266 (incl. Phenobarbital)
Taking anything else?

Check Another of Your Medications Against Theanine

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

From our Q&A

Questions About Theanine or Phenobarbital

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Theanine or Phenobarbital, 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 Phenobarbital

This page covers Theanine only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Phenobarbital page lists 174 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: 1,4-butanediol (major), Activated Charcoal (major), Beer (major), Gamma-hydroxybutyrate (ghb) (major) and Henbane (major).

By severity: 10 major · 143 moderate · 21 minor.

Open the Phenobarbital interaction page

One more smart check

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

We hold 3,298 real supplement products containing Theanine, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Theanine. Your product may carry other ingredients that interact with Phenobarbital too, on top of Theanine — 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 Theanine and Phenobarbital 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 Theanine
Top 5%

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

29 days ago today
Medication Phenobarbital
Top half

#1,135 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 Theanine

What is Theanine?

Theanine
  • Anxiety and stress
  • Relaxation and calm
  • Sleep support
  • Focus and cognitive function

Theanine (usually L-theanine) is an amino acid found naturally in tea leaves that many people take to feel calmer and less stressed without strong drowsiness. Early research suggests it may help with relaxation and stress, but evidence for most uses is still limited. It appears generally well tolerated for short-term use, but check with your pharmacist or doctor before starting it.

Read the full Theanine monograph & all interactions
Medication

About Phenobarbital

What Is Phenobarbital Used For?

Brand names: Phenobarbital, Solfoton, Luminal

Phenobarbital tablets and oral solutions are used to treat generalized and partial seizures and as a sedative. Phenobarbital Sodium injection is used as a long-term anticonvulsant for generalized tonic-clonic and focal seizures, for emergency control of acute convulsive episodes (including status epilepticus), for sedation, short-term treatment of insomnia, and as a preanesthetic. It is also used in pediatric patients as an anticonvulsant and sedative, including before and after surgery.

Sezaby (phenobarbital injection, intravenous) is specifically indicated for the treatment of neonatal seizures in term and preterm infants. It is not approved for use in adolescents or adults.

See full Phenobarbital drug information
Keep checking

Other Drugs & Theanine

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

0Major
172Moderate
392Minor
2,265No known interaction
172 Moderate 392 Minor 2,265 No interaction
Moderate

172 interactions

Showing 60 of 172 moderate interactions — see all 172 on the Theanine page.

Minor

392 interactions

Showing 60 of 392 minor interactions — see all 392 on the Theanine page.

No interaction 2,265 drugs checked — show the list

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

Showing the 60 most looked-up of 2,265 medications checked with no known interaction — see the full Theanine list.

See the full Theanine interactions page
See every supplement checked against Phenobarbital, 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
Theanine monograph › Uses, safety, and every drug interaction in one place Phenobarbital supplement interactions › Every supplement checked against Phenobarbital, ranked by severity Phenobarbital 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 Theanine and Phenobarbital Together: Common Questions

Can you take Theanine with Phenobarbital?
There is only a minor interaction, which is usually not serious. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Theanine and Phenobarbital interaction?
We rate it Minor. This interaction is generally not clinically significant, but it remains worth noting and discussing with your pharmacist.

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

Still Have a Question About Phenobarbital and Theanine?

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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.