Drug & supplement interaction

Guarana + Cimetidine

Cimetidine brand names: Tagamet HB, Tagamet, Cimetidine Injection.

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

Cimetidine falls into 2 drug categories that Guarana can interact with; the most serious is rated moderate. Theoretically, concomitant use might increase the effects and adverse effects of caffeine in guarana.

Read the full interaction report
SeverityModerate
LikelihoodLikely
Interactions2
EvidenceB
What do Severity, Likelihood & Evidence mean?
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Interaction report

Guarana and Cimetidine

These two interact in 2 distinct ways. Each one is explained separately below.

Why Are There 2 Interactions?

Guarana interacts with whole categories of medication. Cimetidine falls into 2 of those categories, so it can interact with Guarana in 2 different ways. We’ve listed each one below — your overall risk is driven by the most serious of them (Moderate).

Moderate

CIMETIDINE (Tagamet)

1 of 2
Theoretically, concomitant use might increase the effects and adverse effects of caffeine in guarana.
Guarana contains caffeine. Cimetidine decreases the rate of caffeine clearance by 31% to 42%.
Severity Moderate Likelihood Likely Evidence B

Well-controlled human studies have demonstrated the likely existence of this interaction.

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

Here's the key thing to know: guarana is basically concentrated caffeine, and cimetidine slows down how fast your body clears caffeine (studies show roughly a 31% to 42% drop in clearance). So the caffeine isn't getting metabolized as quickly, which means it hangs around longer and stacks up. Cimetidine itself isn't being changed here, it's the caffeine in the guarana that builds up.

In real life, that can feel like more of the usual caffeine effects: jitteriness, a racing or pounding heart, trouble sleeping, anxiety, or an upset stomach. If you take Tagamet regularly and enjoy guarana energy products, you may notice caffeine hitting harder than expected. Nothing to panic about, but it's worth chatting with your pharmacist or doctor, especially if you have heart issues or anxiety.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. May DC, Jarboe CH, VanBakel AB, Williams WM. Effects of cimetidine on caffeine disposition in smokers and nonsmokers. Clin Pharmacol Ther 1982;31:656-61. PubMed
Minor

CYTOCHROME P450 1A2 (CYP1A2) INHIBITORS

2 of 2

Why this applies to Cimetidine

Cimetidine falls under CYTOCHROME P450 1A2 (CYP1A2) INHIBITORS — the group Guarana interacts with.

Theoretically, concomitant use might increase the levels and adverse effects of caffeine.
Guarana contains caffeine. Caffeine is metabolized by the CYP1A2 enzyme. Theoretically, drugs that inhibit CYP1A2 may decrease the rate of caffeine clearance and increase caffeine levels.
Severity Minor 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 gist: cimetidine (Tagamet) is well known for slowing down a liver enzyme called CYP1A2, which happens to be the same enzyme your body uses to clear caffeine. Since guarana is basically a concentrated source of caffeine, taking the two together can let that caffeine hang around longer than usual.

In plain terms, you might feel the effects of guarana more strongly: jitteriness, a racing heart, trouble sleeping, or feeling wired. The good news is this is rated a minor interaction based mostly on lab data, so it's more of a comfort issue than a danger for most people. If you notice you're extra buzzed, easing back on the guarana usually helps. Still, check with your pharmacist or doctor if you have heart concerns or anxiety.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Hagg S, Spigset O, Mjorndal T, Dahlqvist R. Effect of caffeine on clozapine pharmacokinetics in healthy volunteers. Br J Clin Pharmacol 2000;49:59-63. PubMed
  2. Carrillo JA, Benitez J. Clinically significant pharmacokinetic interactions between dietary caffeine and medications. Clin Pharmacokinet 2000;39:127-53. 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

Guarana Also Interacts With Other Medications

Guarana has 8 major, 461 moderate, 185 minor known drug interactions in our database — the Cimetidine one above is among them. If you take anything else, check it against the full list below.

Major 8 Moderate 461 Minor 185 No known interaction 2,176 (incl. Cimetidine)
Taking anything else?

Check Another of Your Medications Against Guarana

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

From our Q&A

Questions About Guarana or Cimetidine

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

This page covers Guarana only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Cimetidine page lists 95 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Pu-erh Tea (major), St. John's Wort (major), Adrafinil (moderate) and African Wild Potato (moderate).

By severity: 3 major · 63 moderate · 29 minor.

Open the Cimetidine interaction page

One more smart check

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

We hold 1,430 real supplement products containing Guarana, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Guarana. Your product may carry other ingredients that interact with Cimetidine too, on top of Guarana — 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 Guarana and Cimetidine 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 Guarana
Top 10%

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

29 days ago today
Medication Cimetidine
Steady interest

#1,905 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 Guarana

What is Guarana?

Guarana
  • Energy and alertness
  • Reducing fatigue
  • Weight loss support
  • Athletic performance
  • Mental focus

Guarana is an Amazonian seed that is naturally high in caffeine, which explains most of its stimulant and energy effects. While it may give a short-term boost in alertness and reduce fatigue, strong evidence for weight loss or major performance gains is limited, and it carries the same risks as too much caffeine. Talk to your pharmacist or doctor before using it, especially if you have heart problems, anxiety, or are pregnant.

Read the full Guarana monograph & all interactions
Medication

About Cimetidine

What Is Cimetidine Used For?

Brand names: Tagamet HB, Tagamet, Cimetidine Injection

Cimetidine tablets sold over the counter — including brands like Acid Controller, Acid Reducer, Basic Care Acid Reducer, Equate Cimetidine, and cimetidine 200 — are used to relieve heartburn associated with acid indigestion and sour stomach, and to prevent heartburn brought on by eating or drinking certain foods and beverages.

Prescription-strength cimetidine tablets and Cimetidine Hydrochloride Oral Solution are used for more serious conditions: short-term treatment of active duodenal ulcers (ulcers in the upper small intestine) and benign gastric ulcers (stomach ulcers), maintenance therapy to keep duodenal ulcers from coming back, healing of erosive GERD (acid reflux that has damaged the lining of the esophagus), and treatment of conditions that cause the stomach to make too much acid — such as Zollinger-Ellison Syndrome, systemic mastocytosis, and multiple endocrine adenomas.

See full Cimetidine drug information
Keep checking

Other Drugs & Guarana

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

8Major
461Moderate
185Minor
2,175No known interaction
8 Major 461 Moderate 185 Minor 2,175 No interaction
Major

8 interactions

Moderate

461 interactions

Showing 60 of 461 moderate interactions — see all 461 on the Guarana page.

Minor

185 interactions

Showing 60 of 185 minor interactions — see all 185 on the Guarana page.

No interaction 2,175 drugs checked — show the list

We also checked Guarana 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 Guarana list.

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

Can you take Guarana with Cimetidine?
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.
Why are there 2 interactions listed for Guarana and Cimetidine?
Cimetidine belongs to more than one drug category, and Guarana interacts with each of those categories a little differently. We list every one so nothing is missed — your overall risk is driven by the most serious of them.
How serious is the Guarana and Cimetidine 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 Cimetidine and Guarana?

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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.