What Science Says About Quercetin Benefits, Uses, and Side Effects

Quercetin is a flavonoid that has gained attention over the years for its potential health benefits and is widely sold as a dietary supplement. Like other flavonoids, it is known for its proposed antioxidant and anti-inflammatory effects. While there are some promising animal studies and a small number of human studies, this compound remains fairly elusive in its applications for human health. We will review and examine the evidence for quercetin benefits while also highlighting the limitations and precautions associated with this supplement.

What is quercetin?

Flavonoids are a group of natural substances found in many plant-based foods, such as fruits and vegetables. Flavonoids are generally considered to have antioxidant and anti-inflammatory properties. Due to these properties, quercetin can scavenge free radicals, decrease oxidative stress, and is theorized to help protect against cell damage.

The flavonoid quercetin is commonly found in apples, onions, berries, and many other foods. While quercetin can be obtained from many food sources, it is also available as a dietary supplement in powder or capsule form.

Potential health benefits of quercetin

Quercetin is perceived to have some health benefits for the human body. Some individuals have chosen to use it to help with inflammatory conditions such as arthritis, or conditions of the heart.

However, there is still ongoing research that aims to understand how exactly quercetin affects the human body. Below we will review some of the commonly researched areas in human studies.

Inflammation

There are limited clinical studies analyzing the effect of quercetin on inflammation.

In a meta-analysis, focused on C-reactive protein (CRP) levels, a marker of inflammation in the body, the use of quercetin did appear to lower CRP levels. In another meta-analysis of human clinical trials examining three different inflammatory biomarkers, including CRP, quercetin did not have an overall effect on CRP or the other inflammatory biomarkers evaluated. However, quercetin was found to reduce the amount of CRP in individuals diagnosed with certain health conditions.

Another meta-analysis assessed the effects of quercetin on inflammatory markers in individuals with metabolic syndrome. Once again, CRP levels were reduced with quercetin supplementation, but there was no change in the levels of the other inflammatory markers.

Additionally, a small study reviewed the effect of quercetin on women with rheumatoid arthritis, where 50 women with rheumatoid arthritis received 500 mg of quercetin per day for 8 weeks. Participants reported that supplementation reduced early morning stiffness, morning pain, and after-activity pain.

As we can see, the research surrounding quercetin’s ability to lower inflammatory biomarker levels and improve symptoms of certain inflammatory diseases is inconsistent. While some results are promising, more research is needed to fully understand if quercetin is beneficial for inflammation.

Heart health

Quercetin has been associated with supporting cardiovascular health in a limited number of studies, but the evidence is not conclusive.

In one meta-analysis of human clinical trials, quercetin was shown to significantly reduce blood pressure, particularly with doses above 500 mg per day. The effect of quercetin on systolic blood pressure — the pressure when your heart is pumping blood to the body — was also evaluated in a small study of 93 individuals classified as being overweight. In the study, participants received 150 mg of quercetin per day for 6 weeks. After the treatment period, systolic blood pressure was significantly reduced in the study group.

In another meta-analysis, quercetin consumption also decreased systolic blood pressure, but it had no effect on diastolic blood pressure (pressure when your heart is resting). More evidence is needed to support the effects of quercetin on high blood pressure.

Quercetin may also have a positive effect on cholesterol levels. In one previously mentioned meta-analysis, quercetin intake did significantly reduce total cholesterol and low-density lipoprotein cholesterol levels in individuals with metabolic syndrome. Additional studies are needed to solidify the clinical relevance of these findings on heart disease.

Exercise performance

There is limited information on the benefit of quercetin for improving exercise performance.

Some clinical studies have reported a marginal improvement in exercise performance, however, many have not been able to prove that. The National Institutes of Health also advises that quercetin has little to no effect on endurance performance or maximal oxygen consumption.

Brain health

The exact mechanism by which quercetin may benefit brain health is not fully understood.

However, some studies have suggested that it may provide neuroprotection for individuals with health conditions such as Alzheimer’s and Parkinson’s diseases. The two health conditions are closely associated with neuroinflammation, and quercetin has anti-inflammatory and antioxidant properties.

It’s important to note, however, that most of the studies surrounding quercetin and brain health are animal studies. Additional human clinical trials are needed to confirm if quercetin is beneficial for brain disorders.

Safety of quercetin supplementation

In general, most adults can take up to 1 gram of quercetin a day for 3 months. However, the safety of long-term use has not been established.

Quercetin can be purchased as a tablet, capsule, or powder supplement form, and research is still ongoing to develop other dosage forms that may enhance quercetin absorption in the body.

The FDA has generally recognized quercetin as safe (GRAS). If a product is GRAS, it has been deemed safe by the appropriate experts for its intended use. From the perspective of the FDA, quercetin is intended to be used as an ingredient in beverages like fruit juices and foods such as pasta and processed fruits. The amount of quercetin per serving could be up to 500 mg.

While generally considered safe, not much is known about the potential side effects of quercetin. Quercetin also interacts with several medications. Review below for more information.

Drug	Quinolone antibiotics	Cyclosporine	Diclofenac	Losartan	Midazolam	Pravastatin	Warfarin	Quetiapine	Prazosin	Sulfasalazine
Effect of quercetin on the drug	May decrease the effect of these drugs.	May increase the effects and side effects.	May decrease how fast the drug is cleared from the body, leading to increased side effects.	May change how the drug works in the body.	May reduce the therapeutic effects of the drug.	May decrease the time it takes the drug to clear the body which could lead to increased side effects.	May enhance the effects of the drug which could increase an individual’s risk of bleeding and bruising.	May cause these drugs to stay in the body longer, leading to increased side effects from the medications.	May cause these drugs to stay in the body longer, leading to increased side effects from the medications.	May cause these drugs to stay in the body longer, leading to increased side effects from the medications.

Additionally, if you take medications for diabetes, monitor your blood sugar closely. Quercetin has the potential to lower blood sugar levels, so it is very important to speak with your doctor first before trying any new supplements.

Choosing the right quercetin supplement

Before purchasing a supplement, there are a few things to consider.

Dietary supplements are not required to be approved by the FDA before being sold to consumers. However, by ensuring that manufacturers follow the Current Good Manufacturing Practice (CGMP) Regulations, the FDA can help ensure supplement quality. For this reason, consider purchasing from manufacturers that follow CGMP regulations.

As a pharmacist, I help many individuals in choosing dietary supplements to meet their needs. I also consider patient-specific factors that may influence supplementation, such as drug interactions and underlying medical conditions. Above all, I encourage individuals to purchase high-quality supplements. I have listed a few practical tips below that align with recommendations provided by the FDA:

Avoid supplements with false claims to cure a disease or health condition. According to the FDA, it is not legal for a dietary supplement to be marketed to treat, prevent, or cure a specific disease.

• Avoid supplements advertised to be a ‘quick fix’ or ‘solution.’
• Be aware of fillers. Some supplements also contain other ingredients that can lead to adverse effects.

Be sure to read the product labels in their entirety and speak to your healthcare provider before starting supplementation with quercetin.

Final thoughts

Quercetin has been studied for its potential health benefits, however, more human studies are needed to confirm whether and to what extent quercetin is useful for human health.

Current evidence is inconsistent, and more clinical trials on quercetin would allow us to fill in this knowledge gap.