Quercetin Side Effects, Interactions and Kidney Cautions

Who should ask a clinician or pharmacist first

Ask before using quercetin if you:

• take any prescription medicine, especially blood thinners, blood pressure medicines, heart-rhythm medicines, transplant or cancer medicines (details below);
• are having or planning cancer treatment;
• are pregnant, breastfeeding, or choosing for a child;
• have kidney or liver disease;
• have surgery planned.

Do not stop or change a prescribed medicine on your own; talk to the prescriber.

Side effects reported

1. A 4-week allergy trial (200 mg a day, bioavailable formulation)

   Moderate

   Minor adverse effects only. [2]

2. Oral supplements, clinical studies

   Unclear

   No significant adverse effects at 3 to 1,000 mg a day for up to three months. [1]

3. A trial in 30 men with chronic prostatitis (1,000 mg a day for one month)

   Unclear

   One case of headache and one of tingling in the extremities; both resolved after the study ended. [1]

4. Quercetin given by vein (IV) to cancer patients in a phase I trial

   Unclear

   Nausea, vomiting, sweating, flushing and trouble breathing at doses of 10.5 mg per kg of body weight or more. [1]

The IV result is not about capsules or powders. It is included because it is the only human signal of toxicity in the sources we read.

Kidney cautions

In that same phase I IV trial, higher doses (up to 51.3 mg per kg, about 3,591 mg of quercetin) were associated with kidney toxicity, without signs of infection, inflammation or obstruction. [1] Proper evidence not found for the effect of oral quercetin supplements on kidney function in people. We searched PubMed for kidney and chronic kidney disease studies of quercetin and found only animal or unrelated research. Because the kidneys clear many compounds and the data are missing, people with kidney disease, a transplant or on dialysis should ask their kidney team before using it.

Liver, pregnancy and children

Proper evidence not found in the sources we read for quercetin and liver injury, or for use in pregnancy, breastfeeding or children. Absence of reports is not proof of safety. See liver safety and supplements, pregnancy and breastfeeding and children and teens.

A 2005 review noted that quercetin tested as mutagenic in a bacterial test, that one animal study reported cancer in rats but was not reproduced, and that the International Agency for Research on Cancer concluded in 1999 that it is not classified as carcinogenic to humans. [3] The review was written by a researcher at a drug company, and it is a decades-old reference, so treat it as background only.

Medicine interactions

Most of the interaction information comes from lab or animal studies, and the clinical relevance is often unknown. Memorial Sloan Kettering and the Linus Pauling Institute describe:

1. Medicines processed by the CYP3A4 or CYP2C19 enzymes

   Possible

   Quercetin significantly inhibited these enzymes in the lab; clinical relevance is not known. [4]

2. Medicines moved by P-glycoprotein (for example digoxin, some blood pressure and heart-rhythm medicines, some cancer and transplant medicines)

   Possible

   Very high or supplemental intakes could potentially increase the toxicity of such drugs. [1]

3. Warfarin

   Possible

   A case report of a 79-year-old man on stable warfarin whose INR rose after he started quercetin; it normalized after he stopped. [4] In theory, high flavonoid intakes from supplements could raise bleeding risk with anticoagulant or antiplatelet drugs. [1]

4. Losartan (blood pressure)

   Possible

   Quercetin increased exposure to losartan in an animal (mouse) model. [4]

5. Tamoxifen

   Possible

   Quercetin increased tamoxifen's bioavailability in preclinical studies. [4]

6. Chemotherapy

   Possible

   Because of its antioxidant effects, quercetin may interfere with certain chemotherapy drugs. [4] NCCIH also says antioxidants might reduce the effectiveness of some cancer treatments and to check with the team managing your cancer treatment. [5]

7. Iron supplements

   Possible

   The Linus Pauling Institute says that to maximize iron absorption, flavonoid supplements should not be taken at the same time. [1]

Read medication interactions and ask a pharmacist to check a specific product against your medicines. Longevity-focused quercetin products are discussed on longevity compounds and medication interactions.

Before surgery

Because of the possible bleeding interaction, tell the surgical and anesthesia team about all supplements; see supplements before surgery.

Product quality

Supplements vary in what they contain; see reading supplement labels and quality testing and certifications. If you notice a rash, swelling, trouble breathing or fainting after any supplement, get urgent care (911 in the US).