Introduction
Quercetin is a naturally occurring flavonoid compound found in various fruits, vegetables, and grains. As a dietary supplement, it has gained significant attention for its potential health benefits and pharmacological properties. While not approved as a pharmaceutical drug by regulatory agencies, quercetin is widely available as a nutritional supplement and has been extensively studied for its antioxidant, anti-inflammatory, and potential therapeutic effects.
Mechanism of Action
Quercetin exerts its effects through multiple mechanisms:
- Antioxidant activity: Scavenges free radicals and reactive oxygen species through its phenolic structure
- Anti-inflammatory effects: Modulates inflammatory pathways by inhibiting cyclooxygenase (COX) and lipoxygenase (LOX) enzymes
- Enzyme modulation: Inhibits various enzymes including xanthine oxidase, protein kinase C, and phospholipase A2
- Cell signaling modulation: Affects numerous intracellular signaling pathways including NF-κB and MAPK pathways
- Histamine inhibition: Acts as a mast cell stabilizer and inhibits histamine release
- Metal chelation: Binds to metal ions, potentially reducing oxidative stress
Indications
Note: Quercetin is not FDA-approved for any medical indications. The following uses are based on preliminary research and traditional use:- Dietary supplement for general antioxidant support
- Potential adjunctive support for inflammatory conditions
- Investigationally studied for cardiovascular health support
- Research interest in metabolic syndrome and allergy management
- Exploration in cancer prevention and supportive care contexts
Dosage and Administration
Typical supplemental doses: 500-1000 mg daily, divided into 2-3 doses Higher investigational doses: Up to 2000 mg daily under medical supervision Administration: Preferably with meals to enhance absorption Special populations:- Elderly: No specific dosage adjustments established
- Renal impairment: Use with caution due to limited safety data
- Hepatic impairment: Use with caution due to limited safety data
- Pregnancy and lactation: Not recommended due to insufficient safety data
Pharmacokinetics
Absorption: Poor and variable bioavailability (approximately 2-20%) due to low water solubility and extensive metabolism Distribution: Widely distributed throughout tissues; crosses the blood-brain barrier Metabolism: Extensive first-pass metabolism via phase II enzymes (glucuronidation, sulfation, methylation) Elimination: Primarily renal excretion of metabolites; half-life approximately 3.5 hours Factors affecting bioavailability: Concomitant food intake, formulation (liposomal or phospholipid complexes may enhance absorption), and individual variation in gut microbiotaContraindications
- Known hypersensitivity to quercetin or related flavonoids
- Pregnancy and breastfeeding (due to insufficient safety data)
- Severe renal or hepatic impairment (theoretical risk due to limited elimination data)
Warnings and Precautions
- May interact with numerous medications (see Drug Interactions section)
- Theoretical risk of nephrotoxicity at high doses
- Potential for allergic reactions in sensitive individuals
- Quality control concerns with dietary supplements (variability in potency and purity)
- Long-term safety data limited beyond 12 weeks of continuous use
Drug Interactions
Cytochrome P450 interactions:- Inhibits CYP3A4, CYP2C9, and CYP2D6 (may increase levels of substrates)
- Potential interaction with drugs metabolized by these enzymes
- Inhibits P-glycoprotein (may affect digoxin, cyclosporine, and other P-gp substrates)
- Anticoagulants: Theoretical increased bleeding risk
- Chemotherapy drugs: May alter efficacy (consult oncologist)
- Antibiotics: May affect fluoroquinolone absorption
- Immunosuppressants: Potential interaction with cyclosporine and tacrolimus
- Vitamin C: May enhance absorption and effects
- Bromelain: Often combined to potentially enhance absorption
Adverse Effects
Common effects (typically mild and dose-related):- Headache
- Gastrointestinal discomfort (nausea, heartburn)
- Peripheral tingling sensation
- Allergic reactions (rash, urticaria)
- Renal toxicity (with very high doses)
- Hepatotoxicity (case reports with high doses)
- Pro-oxidant effects at very high concentrations
- Hormonal effects due to weak estrogenic activity
Monitoring Parameters
- Renal function (BUN, creatinine) with long-term high-dose use
- Liver function tests with prolonged high-dose supplementation
- Clinical assessment for efficacy and adverse effects
- Medication levels for drugs with narrow therapeutic indices
- Bleeding parameters if used with anticoagulants
Patient Education
- Quercetin is a dietary supplement, not an FDA-approved medication
- Inform healthcare providers about all supplement use
- Purchase from reputable manufacturers that provide third-party testing
- Be aware of potential interactions with prescription medications
- Discontinue use 2 weeks before elective surgery due to potential bleeding risk
- Report any unusual symptoms or side effects to healthcare provider
- Understand that evidence for many claimed benefits is preliminary
- Do not use as replacement for prescribed medications without medical consultation
References
1. Boots AW, et al. Health effects of quercetin: from antioxidant to nutraceutical. Eur J Pharmacol. 2008;585(2-3):325-337. 2. Li Y, et al. Quercetin, inflammation and immunity. Nutrients. 2016;8(3):167. 3. Andres S, et al. Safety aspects of the use of quercetin as a dietary supplement. Mol Nutr Food Res. 2018;62(1):1700447. 4. D'Andrea G. Quercetin: A flavonol with multifaceted therapeutic applications? Fitoterapia. 2015;106:256-271. 5. National Institutes of Health Office of Dietary Supplements. Quercetin Fact Sheet for Health Professionals. 6. European Food Safety Authority. Scientific Opinion on the safety of quercetin. EFSA Journal. 2011;9(4):2067. 7. Kelly GS. Quercetin. Altern Med Rev. 2011;16(2):172-194. 8. ClinicalTrials.gov. Search results for quercetin clinical trials.
This monograph is for educational purposes only and does not constitute medical advice. Consult a healthcare professional before starting any new supplement regimen.