Quercetin

What it is

Quercetin is a flavonoid pigment found in onions, apples, berries, capers and tea. Most people get some from their diet.

Supplements are marketed for allergy symptoms, immune support, blood pressure and, more recently, as a "senolytic" alongside dasatinib in longevity research.

The senolytic work is experimental and involves a prescription cancer drug; it does not mean quercetin alone does the same thing.

How it works

Quercetin is an antioxidant that can also influence inflammatory signalling and stabilise mast cells in lab studies. Absorption from plain supplements is low, so blood levels remain modest.

What the evidence says

• Moderate Blood pressure
  Meta-analyses suggest a small reduction at doses of about 500 mg or more a day, mainly in people with raised blood pressure.
• Limited Exercise or physical performance
  Small effects on endurance in some trials; not a reliable performance aid.
• Weak Allergy symptoms
  Mostly lab and small studies; not proven to replace antihistamines.
• Limited Cold and respiratory infection
  Some trials show slight reductions in symptoms, especially under physical stress.
• Weak Senolytic or anti-ageing effect
  Mostly cell and animal work, and human studies use combinations with a prescription drug.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Typical doses are 250-500 mg once or twice daily.
• Take with food; fat can help absorption.
• Some people cycle it (for example, several weeks on, then off), but there is no firm evidence for a best schedule.
• No official recommended intake; check current guidance and the label.

Forms and what to look for

• Quercetin dihydrate is cheaper and better absorbed than the anhydrous form.
• Phytosome (bound to lecithin) and other enhanced forms claim better bioavailability.
• Products with vitamin C and bromelain are common; extra benefit is not well proven.
• Check the label states mg of quercetin per serving.

What to expect, and when

• Blood pressure effects in trials appeared over about 4-12 weeks.
• Allergy or immune effects, if any, are subtle.
• Many people will not notice a difference.

Side effects

• Headache or tingling in the limbs reported at higher doses.
• Stomach upset or nausea.
• Kidney concerns at very high doses have been seen in animals.
• Rarely, allergic reactions.

Interactions

• Antibiotics (quinolones such as ciprofloxacin): quercetin may reduce their absorption or effect.
• Blood thinners such as warfarin: may increase bleeding risk.
• Drugs broken down by liver enzymes, including ciclosporin: levels can change; check with a pharmacist.
• Blood pressure medicines: effects may add together.
• Chemotherapy: check with your oncology team.

Who should avoid it

• Pregnant or breastfeeding women using supplement doses.
• People with kidney disease unless a clinician agrees.
• People on anticoagulants, ciclosporin or quinolone antibiotics unless advised.
• Anyone preparing for surgery; stop as advised.

Risks and safety

• Long-term high-dose safety is not well studied.
• Drug interactions via liver enzyme effects are the main practical risk.
• Senolytic protocols involving prescription drugs should only be done in clinical trials or under medical supervision.
• Quality varies between brands.

Buying well: quality and testing

• Look for third-party tested products showing quercetin content.
• Choose clear single-ingredient products over proprietary blends.
• Be wary of claims that quercetin "clears old cells" on its own.
• Check whether the dose listed is quercetin or total flavonoids.

Bottom line

Quercetin has a reasonable, modest case for blood pressure and weaker support elsewhere. Its drug interactions deserve respect. The senolytic buzz is early-stage science, not a reason to take supplements for anti-ageing. It suits adults without relevant medicines who want a low-cost, low-expectation trial.