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Quercetin

Supplement flavonoid immuneheart & metabolic
Aka: quercetin dihydrate, quercetin phytosome
Brands seen in Singapore: California Gold Nutrition, NOW Foods, Doctor's Best, Thorne, Life Extension
Third-party tested: Thorne certifies other products with NSF Certified for Sport, but nothing for quercetin.Certification is granted per product, so check the tub you actually own. Not being listed is not the same as failing.
Not medical advice. Check with your doctor or pharmacist before you start, stop or change the timing of anything you take.
How to use
What it’s for
Unclear — immune data is in animals, heart data is small
Take with
Sold with bromelain in SG · no evidence found for that pair
Studied for
Trials ran 1 to 8 weeks · nothing longer has been studied
Pros & cons
Pros
  • ODS: no side effects at 1,000 mg/day for 2 months
  • FDA: up to 500 mg per serving is gras in foods
  • Human trials exist — 17 RCTS, 896 people, pooled
Cons
  • Immune evidence is preclinical — animals, not people
  • ODS: performance effect trivial to none, inconsistent
  • Safety above 1,000 mg, or past 2 months, is unstudied
Sources
  1. NIH ODS — Dietary Supplements for Exercise and Athletic Performance, Quercetin (Efficacy); ods.od.nih.gov returned Cloudflare 403 to a direct fetch, so the 1 Aug 2026 archive.org capture was read · read 2026-08-27
    what it says
    “These studies typically examined the endurance performance and VO2max of participants engaged in aerobic activities … who received either quercetin (most often 1,000 mg/day) or placebo for 1 to 8 weeks”
  2. NIH ODS — Exercise and Athletic Performance, Quercetin (what the trials showed) (archive.org capture, 1 Aug 2026) · read 2026-08-27
    what it says
    “The effects of quercetin supplementation were inconsistent and varied by study, but they generally ranged from no ergogenic benefit to only a trivial or small improvement that might not be meaningful in real-world … exercise conditions”
  3. NIH ODS — Exercise and Athletic Performance, Quercetin (Safety) (archive.org capture, 1 Aug 2026) · read 2026-08-27
    what it says
    “took as much as 1,000 mg/day quercetin for up to 2 months found no side effects or safety concerns. The safety of longer term use of that amount of quercetin or more has not been studied.”
  4. NIH ODS — Exercise and Athletic Performance, Quercetin (FDA GRAS status in foods) (archive.org capture, 1 Aug 2026) · read 2026-08-27
    what it says
    “FDA considers up to 500 mg/serving quercetin to be generally recognized as safe (GRAS) as an ingredient in foods such as beverages, processed fruits and fruit juices, grain products, pastas, and soft candies”
  5. NIH ODS — Exercise and Athletic Performance, Quercetin (dietary intake, NHANES 1999–2002) (archive.org capture, 1 Aug 2026) · read 2026-08-27
    what it says
    “An analysis of NHANES 1999–2002 data found that estimated daily total flavonol (including quercetin) consumption by adults averaged about 13 mg/day”
  6. Effect of quercetin supplementation on plasma lipid profiles, blood pressure, and glucose levels: a systematic review and meta-analysis (Nutrition Reviews 2020; PMID 31940027) · read 2026-08-27
    what it says
    “Clinical trials examining the cardiovascular protective effects of quercetin in humans have reported conflicting results … Seventeen trials (n = 896 participants total) were included … Neither lipid profiles nor glucose concentrations changed significantly.”
  7. Effectiveness of supplementation with quercetin-type flavonols for treatment of viral lower respiratory tract infections: systematic review and meta-analysis of PRECLINICAL studies (Phytotherapy Research 2021; PMC8250479) · read 2026-08-27
    what it says
    “we conducted a systematic review to describe the use of quercetin-type flavonols against lower respiratory tract viruses and discussed the preclinical impact … significantly reduced mortality rate … of infected animals”
  8. HSA Singapore — Guidelines on Prohibited and Restricted Ingredients in HS and TM, GL-CHPB-1-012 Rev. No. 001 (October 2025), clause 1.3 + Annexes A and B · read 2026-08-27
    what it says
    “The objective of this guideline is to highlight the substances of concern when used in HS and TM and is not meant to be an exhaustive list [quercetin not listed in Annex A or B]”
  9. iHerb Singapore — Quercetin category, default order (1 - 48 of 145 results) · read 2026-08-27
    what it says
    “California Gold Nutrition, NMN Complex with trans-Resveratrol, Quercetin Phytosome Complex … NOW Foods, Quercetin with Bromelain … NOW Foods, Quercetin, 500 mg … Doctor's Best, Quercetin Bromelain … Thorne, Quercetin Phytosome … Life Extension, Bio-Quercetin®”
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Updated 2026-08-30. Every line above traces to a numbered source. Kozen-X is not a medical device and does not give medical advice. It lists what published sources have written down — nothing more. A pair not listed is unrecorded, not cleared. Before you start, stop or re-time anything, talk to your doctor or pharmacist.