Drug & supplement interaction

Quercetin + Fosinopril, Hydrochlorothiazide

Fosinopril, Hydrochlorothiazide brand name: Monopril HCT.

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

Fosinopril, Hydrochlorothiazide falls into 3 drug categories that Quercetin can interact with; the most serious is rated moderate. Theoretically, taking quercetin with antihypertensive drugs might increase the risk of hypotension.

Read the full interaction report
SeverityModerate
LikelihoodPossible
Interactions3
EvidenceB
What do Severity, Likelihood & Evidence mean?
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Interaction report

Quercetin and Fosinopril, Hydrochlorothiazide

These two interact in 3 distinct ways. Each one is explained separately below.

Why Are There 3 Interactions?

Quercetin interacts with whole categories of medication. Fosinopril, Hydrochlorothiazide falls into 3 of those categories, so it can interact with Quercetin in 3 different ways. We’ve listed each one below — your overall risk is driven by the most serious of them (Moderate).

Moderate

ANTIHYPERTENSIVE DRUGS

1 of 3

Why this applies to Fosinopril, Hydrochlorothiazide

Fosinopril, Hydrochlorothiazide falls under ANTIHYPERTENSIVE DRUGS — the group Quercetin interacts with.

Theoretically, taking quercetin with antihypertensive drugs might increase the risk of hypotension.

Quercetin can modestly decrease blood pressure in people with mild hypertension. Theoretically, it might have additive blood pressure lowering effects when used with antihypertensive drugs.
Severity Moderate Likelihood Possible Evidence B

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

Your Monopril HCT combines two blood pressure medicines: fosinopril (an ACE inhibitor) and hydrochlorothiazide (a water pill). Both already work to lower your blood pressure. Quercetin, in higher supplement doses, has shown a small blood pressure lowering effect of its own in some studies. So the concern here is simple: stacking quercetin on top of your medicine could nudge your blood pressure a little lower than intended.

If that happens, you might feel lightheaded, dizzy, tired, or woozy when you stand up. Keep in mind this is mostly theoretical and based on limited research, and quercetin from everyday foods like onions and apples is very unlikely to matter. Concentrated supplements are the real question. Before starting one, check in with me or your doctor so we can keep an eye on your numbers.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Edwards RL, Lyon T, Litwin SE, et al. Quercetin reduces blood pressure in hypertensive subjects. J Nutr 2007;137:2405-11.
  2. Larson A, Witman MA, Guo Y, et al. Acute, quercetin-induced reductions in blood pressure in hypertensive individuals are not secondary to lower plasma angiotensin-converting enzyme activity or endothelin-1: nitric oxide. Nutr Res. 2012;32(8):557-64. PubMed
Moderate

ORGANIC ANION TRANSPORTER 3 (OAT3) SUBSTRATES

2 of 3

Fosinopril, Hydrochlorothiazide also falls under ORGANIC ANION TRANSPORTER 3 (OAT3) SUBSTRATES.

Theoretically, concomitant use might increase the effects and adverse effects of OAT3 substrates.

In vitro research shows that quercetin is a strong non-competitive inhibitor of OAT3, with half-maximal inhibitory concentration (IC50) values as low as 0.75 mcM. So far, this interaction has not been reported in humans.
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

Your blood pressure pill is actually two medicines in one: fosinopril (an ACE inhibitor) and hydrochlorothiazide (a water pill). Both of these are partly cleared from your body by a kidney transporter called OAT3. In test-tube studies, quercetin blocks that transporter quite strongly. The theory is that if quercetin slows this clearance, a bit more of the medicine could linger in your system.

What that might look like in real life: slightly stronger effects, like dizziness, lightheadedness when standing, low blood pressure, or shifts in potassium or sodium from the water pill side. That said, this is only seen in lab studies and has never been reported in actual people, so I wouldn't lose sleep over it. Food amounts of quercetin are very unlikely to matter. If you're considering high-dose capsules, just run it by your pharmacist or doctor first.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Li C, Wang X, Bi Y, et al. Potent Inhibitors of Organic Anion Transporters 1 and 3 From Natural Compounds and Their Protective Effect on Aristolochic Acid Nephropathy. Toxicol Sci. 2020;175(2):279-291. PubMed
  2. Ni Y, Duan Z, Zhou D, et al. Identification of Structural Features for the Inhibition of OAT3-Mediated Uptake of Enalaprilat by Selected Drugs and Flavonoids. Front Pharmacol. 2020;11:802. PubMed
Moderate

ORGANIC ANION TRANSPORTER 1 (OAT1) SUBSTRATES

3 of 3

Fosinopril, Hydrochlorothiazide also falls under ORGANIC ANION TRANSPORTER 1 (OAT1) SUBSTRATES.

Theoretically, concomitant use might increase the effects and adverse effects of OAT1 substrates.

In vitro research shows that quercetin is a strong non-competitive inhibitor of OAT1, with half-maximal inhibitory concentration (IC50) values less than 10 mcM. So far, this interaction has not been reported in humans.
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 gist for your Monopril HCT (fosinopril plus hydrochlorothiazide). Your kidneys use a little doorway called OAT1 to help clear certain medicines out through the urine. In lab dish studies, quercetin seems to slow that doorway down. If that happened in real life, these two blood pressure ingredients could hang around a bit longer than usual, which in theory might nudge their effects up a notch (more blood pressure lowering, light-headedness, more frequent urination, or shifts in potassium and sodium).

I want to be honest though: this is only test-tube evidence, and it hasn't actually been seen in people. Everyday food amounts of quercetin are very unlikely to matter. Before starting a high-dose supplement, just run it by your pharmacist or doctor.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Ni Y, Duan Z, Zhou D, et al. Identification of Structural Features for the Inhibition of OAT3-Mediated Uptake of Enalaprilat by Selected Drugs and Flavonoids. Front Pharmacol. 2020;11:802. 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

Quercetin Also Interacts With Other Medications

Quercetin has 1,168 moderate known drug interactions in our database — the Fosinopril, Hydrochlorothiazide one above is among them. If you take anything else, check it against the full list below.

Moderate 1,168 No known interaction 1,662 (incl. Fosinopril, Hydrochlorothiazide)
Taking anything else?

Check Another of Your Medications Against Quercetin

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

Everything else you take

See Every Supplement That Interacts With Fosinopril, Hydrochlorothiazide

This page covers Quercetin only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Fosinopril, Hydrochlorothiazide page lists 238 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), American Hellebore (major), American Mistletoe (major), Asarabacca (major) and Dolomite (major).

By severity: 11 major · 200 moderate · 27 minor.

From our pharmacist’s overview on the Fosinopril, Hydrochlorothiazide page

“Monopril HCT combines two blood pressure medicines, the ACE inhibitor fosinopril and the water pill hydrochlorothiazide, which is why I read its supplement list with genuine wariness rather than a shrug. Eleven of the 238 ingredients on file carry a major rating and deserve your attention first, while most of the 200 rated moderate eit…”

Reviewed by 2 · Jul 18, 2026

Open the Fosinopril, Hydrochlorothiazide interaction page

One more smart check

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

We hold 4,589 real supplement products containing Quercetin, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Quercetin. Your product may carry other ingredients that interact with Fosinopril, Hydrochlorothiazide too, on top of Quercetin — 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 Quercetin and Fosinopril, Hydrochlorothiazide 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 Quercetin
Top 5%

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

29 days ago today
Medication Fosinopril, Hydrochlorothiazide
Top 5%

#102 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 Quercetin

What is Quercetin?

Quercetin
  • Seasonal allergies
  • Antioxidant support
  • Heart and blood pressure health
  • Anti-inflammatory support
  • Immune support
  • Exercise recovery

Quercetin is a plant flavonoid with antioxidant and anti-inflammatory properties found in many common foods and sold as a supplement. While early research is interesting for allergies, blood pressure, and inflammation, strong human evidence is limited for most uses. It is generally well tolerated in food amounts, but talk to your pharmacist or doctor before taking high-dose supplements.

Read the full Quercetin monograph & all interactions
Keep checking

Other Drugs & Quercetin

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

0Major
1,168Moderate
0Minor
1,661No known interaction
1,168 Moderate 1,661 No interaction
Moderate

1,168 interactions

Showing 60 of 1,168 moderate interactions — see all 1,168 on the Quercetin page.

No interaction 1,661 drugs checked — show the list

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

Showing the 60 most looked-up of 1,661 medications checked with no known interaction — see the full Quercetin list.

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

Can you take Quercetin with Fosinopril, Hydrochlorothiazide?
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 3 interactions listed for Quercetin and Fosinopril, Hydrochlorothiazide?
Fosinopril, Hydrochlorothiazide belongs to more than one drug category, and Quercetin 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 Quercetin and Fosinopril, Hydrochlorothiazide 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

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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.