Nutritional Supplement


  • Negative Interactions

    • Quercetin


      Potential Negative Interaction

      Studies have shown that grapefruit juice significantly increases estradiol levels in the blood. One of the flavonoids found in grapefruit juice is quercetin. In a test tube study, quercetin was found to change estrogen metabolism in human liver cells in a way that increases estradiol levels and reduces other forms of estrogen. This effect is likely to increase estrogen activity in the body. However, the levels of quercetin used to alter estrogen metabolism in the test tube were much higher than levels found in the body after supplementing with quercetin.

      There is evidence from test tube studies that another flavonoid in grapefruit juice, naringenin, also has estrogenic activity. It has yet to be shown that dietary or supplemental levels of quercetin (or naringenin) could create a significant problem.

      1. Schubert W, Cullberg G, Edgar B, Hedner T. Inhibition of 17 beta-estradiol metabolism by grapefruit juice in ovariectomized women. Maturitas 1994;20:155-63.
      2. Weber A, Jager R, Borner A, et al. Can grapefruit juice influence ethinylestradiol bioavailability? Contraception 1996;53:41-7.
      3. Schubert W, Eriksson U, Edgar B, et al. Flavonoids in grapefruit juice inhibit the in vitro hepatic metabolism of 17 beta-estradiol. Eur J Drug Metab Pharmacokinet 1995;3:219-24.
      4. Kuiper GG, Lemmen JG, Carlsson B, et al. Interaction of estrogenic chemicals and phytoestrogens with estrogen receptor beta. Endocrinology 1998;139:4252-63.
    • Quercetin


      Potential Negative Interaction

      Quercetin is a flavonoid found in grapefruit juice, tea, onions, and other foods; it is also available as a nutritional supplement. Quercetin has been shown in test tube studies to inhibit enzymes responsible for breaking down felodipine into an inactive form. This interaction may result in increased blood levels of felodipine that could lead to unwanted side effects. Until more is known about this interaction, patients taking felodipine should avoid supplementing with quercetin.

      1. Miniscalco A, Lundahl J, Regardh CG. Inhibition of dihydropyridine metabolism in rat and human liver microsomes by flavonoids found in grapefruit juice. J Pharmacol Exp Ther 1992;261:1195-9.
  • Explanation Required

    • Quercetin


      Needs Explanation

      In an animal study, oral administration of quercetin (50 mg per 2.2 pounds of body weight) at the same time as cyclosporine decreased the absorption of cyclosporine by 43%. However, in a study of healthy human volunteers, supplementing with quercetin along with cyclosporine significantly increased blood levels of cyclosporine, when compared with administering cyclosporine alone. Because the effect of quercetin supplementation on cyclosporine absorption or utilization appears to be unpredictable, individuals taking cyclosporine should not take quercetin without the supervision of a doctor.

      1. Hsiu SL, Hou YC, Wang YH, et al. Quercetin significantly decreased cyclosporin oral bioavailability in pigs and rats. Life Sci 2002;72:227-35.
      2. Choi JS, Choi BC, Choi KE. Effect of quercetin on the pharmacokinetics of oral cyclosporine. Am J Health Syst Pharm 2004;61:2406-9.


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17. Griffith JQ. Clinical application of quercetin: preliminary report. J Am Pharm Assoc 1953;42:68-9.

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19. Yao Z, Gu Y, Zhang Q, et al. Estimated daily quercetin intake and association with the prevalence of type 2 diabetes mellitus in Chinese adults. Eur J Nutr 2019;58:819–30.

20. Chen S, Jiang H, Wu X, Fang J. Therapeutic Effects of Quercetin on Inflammation, Obesity, and Type 2 Diabetes. Mediators Inflamm 2016;2016:9340637.

21. Peng J, Li Q, Li K, et al. Quercetin Improves Glucose and Lipid Metabolism of Diabetic Rats: Involvement of Akt Signaling and SIRT1. J Diabetes Res 2017;2017:3417306.

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25. Shi G, Li Y, Cao Q, et al. In vitro and in vivo evidence that quercetin protects against diabetes and its complications: A systematic review of the literature. Biomed Pharmacother 2019;109:1085–99.

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29. Amoros M, Simoes CM, Girre L, et al. Synergistic effect of flavones and flavonols against herpes simplex virus type 1 in cell culture. Comparison with the antiviral activity of propolis. J Nat Prod 1992;55:1732-40.

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35. Bindoli A, Valente M, Cavallini L. Inhibitory action of quercetin on xanthine oxidase and xanthine dehydrogenase activity. Pharmacol Res Commun 1985;17:831-9.

36. Shi Y, Williamson G. Quercetin lowers plasma uric acid in pre-hyperuricaemic males: a randomised, double-blinded, placebo-controlled, cross-over trial. Br J Nutr 2016;115:800–6.

37. Nieman DC, Henson DA, Gross SJ, et al. Quercetin reduces illness but not immune perturbations after intensive exercise. Med Sci Sports Exerc 2007;39:1561-9.

38. Varma SD, Mizuno A, Kinoshita JH. Diabetic cataracts and flavonoids. Science 1977;195:205.

39. Nieman DC, Henson DA, Gross SJ, et al. Quercetin reduces illness but not immune perturbations after intensive exercise. Med Sci Sports Exerc 2007;39:1561-9.

40. Ishikawa M, Oikawa T, Hosokawa M, et al. Enhancing effect of quercetin on 3-methylcholanthrene carcinogenesis in C57B1/6 mice. Neoplasma 1985;43:435-41.

41. Hertog M, Feskens EJM, Hollman PCH, et al. Dietary flavonoids and cancer risk in the Zutphen Elderly Study. Nutr Cancer 1994;22:175-84.

42. Castillo MH, Perkins E, Campbell JH, et al. The effects of the bioflavonoid quercetin on squamous cell carcinoma of head and neck origin. Am J Surg 1989;351-5.

43. Stavric B. Quercetin in our diet: from potent mutagen to probable anticarcinogen. Clin Biochem 1994;27:245-8.

44. Barotto NN, López CB, Eyard AR, et al. Quercetin enhances pretumourous lesions in the NMU model of rat pancreatic carcinogenesis. Cancer Lett 1998;129:1-6.

45. Stoewsand GS, Anderson JL, Boyd JN, Hrazdina G. Quercetin: a mutagen, not a carcinogen in Fischer rats. J Toxicol Environ Health 1984;14:105-14.

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The information presented by Healthnotes is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. For many of the conditions discussed, treatment with prescription or over the counter medication is also available. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires December 2020.