Quercetin

What is quercetin?

Quercetin is a flavonoid, a type of plant polyphenol, found in onions, apples, berries, capers, kale, tea and red wine. In foods it is usually attached to sugars, for example as rutin, which is quercetin bound to a sugar called rutinose. Supplements most often contain the free form, quercetin aglycone. In laboratory studies quercetin acts as an antioxidant and affects inflammation-related pathways, which has made it popular in immune, allergy, heart health and anti-aging products.

What the research says

Interest in quercetin for eyes comes mainly from laboratory and animal research suggesting it may protect the lens and retina from oxidative stress. There are no strong human trials showing that quercetin supplements improve vision, protect against age-related eye changes or ease eye strain. Human research on quercetin in general has focused on blood pressure, exercise performance, allergies and inflammation markers, with mixed and generally modest results.

On safety, a 2018 review by scientists at the German Federal Institute for Risk Assessment examined quercetin as a single-ingredient supplement. They noted that supplements often provide up to 1,000 mg a day, far more than people get from food. In human studies, side effects were rarely reported and mild. However, the authors found no adequate data on long-term use (beyond 12 weeks) of doses of 1,000 mg or more. Animal studies raised possible concerns about worsening kidney damage in people with existing kidney problems and about promoting estrogen-dependent tumors. Human and animal studies also showed that quercetin can change the bioavailability of certain drugs, which could make them stronger or weaker.

How it’s used in supplements

Quercetin is sold alone, often with vitamin C or bromelain, and is a common minor ingredient in eye, immune and antioxidant blends. In multi-ingredient eye formulas the amount is usually small and undisclosed, which makes any benefit even harder to judge. People with kidney disease, those with a history of hormone-sensitive cancers, pregnant or breastfeeding women, and anyone taking prescription drugs, especially blood thinners or medicines with a narrow safety margin, should talk to a doctor or pharmacist before taking quercetin supplements. Eating onions, apples, berries and tea remains a low-risk way to get quercetin along with other beneficial plant compounds.

In short, quercetin is a well-known antioxidant with an interesting laboratory profile, but there is no direct evidence that it supports vision in people. Its main relevance for shoppers is as a possible source of drug interactions in multi-ingredient formulas, which is worth raising with a pharmacist if you take regular medicines.