Who should not take quercetin?

Quercetin is a flavonoid, a compound that gives plants—including fruits, vegetables, herbs, and flowers—their color. It’s found in many common foods and drinks, such as citrus fruit, apples, onions, buckwheat, cherries, berries, broccoli, red wine, green tea, parsley, sage, dill, and coriander. It’s also available as a supplement. If you’ve noticed it in grocery and drug store aisles recently, it may be because this dietary antioxidant is becoming more popular. 

“There’s good reason for the growing interest,” says Jacob Teitelbaum, MD, an integrative medicine physician in Hawaii and author of From Fatigued to Fantastic.

Quercetin is purported to have many health benefits. For example, it may prevent cardiovascular disease, has anti-inflammatory effects, and may fend off SARS-CoV-2, the virus that causes COVID-19. It’s generally considered safe, but people with certain health conditions should avoid taking it as a supplement. Here’s what you need to know about this flavonoid and its safety profile.

Who should not take quercetin?

Quercetin is safe for most people to consume in food. “It’s also been shown to be safe as a supplement at concentrations when taken at doses lower than 1000 mg,” says William Li, MD, physician and author of Eat to Beat Disease. It’s generally recognized as safe by the U.S. Food and Drug Administration (FDA).

However, experts recommend that the following groups avoid taking quercetin supplements:

• People with kidney disease: High doses of quercetin could damage your kidneys. If your kidneys are already compromised, it could be especially risky.
• People with caffeine sensitivities: Some research suggests that taking quercetin may inhibit the metabolism of caffeine. Taking the supplement might increase the blood levels of caffeine, notes Dr. Li.
• Children and people who are pregnant or breastfeeding: There is no human data proving this supplement’s safety for expectant mothers, a developing fetus, a new baby, or children. Additionally, pregnancy stresses the kidneys. Taking quercetin could “cause kidney problems, especially in someone whose kidney function is already altered, like during pregnancy,” says Emily Neiman, RN, a certified nurse midwife with Flourish Midwifery Care.

Additionally, people taking certain medications should not take quercetin to avoid a drug interaction, such as:

• Blood thinners: Quercetin may increase the risk of bleeding when you’re already taking anticoagulant medication, such as Coumadin (warfarin), notes Dr. Teitelbaum.
• Medications processed by the cytochrome P450 enzyme system: These include the immune system suppressant drug cyclosporine and certain antibiotics, such as ciprofloxacin.
• Blood pressure medications: Because quercetin may also lower blood pressure, you should be cautious about taking any antihypertensive medication and quercetin at the same time.

Always check with a healthcare provider before taking quercetin if you are on any prescription medications.

How does quercetin work?

Research suggests that eating a quercetin-rich diet or taking a quercetin supplement has many health benefits, such as:

• Protecting cells from oxidative stress by fighting off free radical molecules
• Preventing the growth of certain types of cancer cells, including breast, colon, cervical, lung, prostate, and liver cancer
• Helping decrease high blood pressure if you have hypertension and prevent heart disease
• Lowering cholesterol levels and preventing damage from LDL cholesterol
• Keeping blood sugar levels under control
• Stabilizing the cells in your body that release histamine in response to a pathogen and allergen so that it can have an antihistamine effect
• Reducing symptoms of prostatitis, especially when combined with digestive enzymes like bromelain

These benefits are due to its antioxidant, anticancer, and anti-inflammatory properties.

What are the side effects of quercetin?

Like any medication or supplement, there are a couple of potential side effects of taking quercetin, including:

• Headache: Headaches are a common side effect of quercetin. In fact, some experts suggest that quercetin could be the cause of the headache you get after drinking too much red wine. The theory is that the quercetin in the skin of red grapes might create a type of toxic byproduct, and you get a headache as a result.
• Upset stomach: If your stomach feels a little upset after taking quercetin, don’t be too surprised. Another common side effect of quercetin is nausea and vomiting. It might help to take the supplement with food rather than on an empty stomach.

“As with any supplement, if you experience side effects, the best thing to do is stop the supplement and let the side effects subside,” says Dr. Li. “You might then try again at a lower dose to see if the side effects return.”

How to take quercetin safely

Quercetin is an antioxidant that is safe for most people to take as long as you follow a few guidelines. First, be sure to take a daily dosage of 1 gram (1,000 mg) or less. No clinical trials have reported serious adverse events from daily doses of up to 1000 mg of quercetin for up to 12 weeks.

Discuss all supplements and medications you are taking with a healthcare professional or pharmacist before starting quercetin. Some common supplements, such as St. John’s wort and ginkgo biloba, contain quercetin, and several common medications interact with quercetin.

Then, figure out how to incorporate quercetin into your daily routine. The supplement is most effective when taken with meals. Certain foods can aid its absorption, and it prevents upset stomach. It also helps with consistency. Like any supplement, quercetin is only effective if you take it regularly.