Onions (Allium cepa), both white and red, are among the richest sources of quercetin. Other foods such as citrus fruits, apples, parsley, sage, grapes, dark cherries, and berries like blueberries, blackberries, and bilberries also contain notable amounts of quercetin. Tea, especially varieties from the Camellia sinensis plant, is another significant source of this beneficial compound. Herbal teas, like chamomile and elderberry, also provide quercetin, though in smaller amounts.

Quercetin is also present in medicinal plants like Ginkgo biloba, St. John’s Wort (Hypericum perforatum), and black elderberry (Sambucus canadensis). Additionally, olive oil, red wine, and honey are important dietary sources of quercetin, and it can also be taken as a supplement.

• Cardio-protection

Animal studies and clinical data suggest that quercetin has a strong cardio-protective effect. This compound helps lower high blood pressure through its anti-inflammatory and antioxidant properties. Additionally, quercetin may aid in restoring the strength of heart muscle that can be weakened by cardiac conditions such as heart failure, diabetic cardiomyopathy, and ischemic heart disease.

• Anti-inflammation

Quercetin is known for its long-lasting anti-inflammatory effects. It works on different types of cells in both animals and humans. It helps stabilize mast cells and protect the digestive system. Quercetin also helps control inflammation and immune responses in a balanced way. Additionally, it can suppress the activity of dendritic cells, which are important for the immune system.

• Anti-cancer

Owing to its extensive antioxidant activity, quercetin is capable of directly attacking human cancer cells, initiating their death, and consequently inhibiting the progression of cancerous tissue. Despite its detrimental effect on cancerous cells, this compound shows minimal toxicity to normal human cells, even at high doses.

• Antiviral activity

Evidence suggests that quercetin may be beneficial for the treatment of viral infections, as quercetin-type flavonols exhibit direct antiviral activity and indirect anti-inflammatory effects. These compounds can significantly reduce the mortality rate during viral upper respiratory infections. Interestingly, quercetin was one of the natural compounds studied for its potential against coronavirus infection, as it has been shown to inhibit viral replication.

• Allergy

Since allergies to certain plants or their pollen are quite common, it is crucial to first test for any allergic reaction to this compound.

• Drug interactions

Generally, quercetin interacts with several medications that are metabolized in the liver, transported by P-glycoprotein pumps, or excreted through the kidneys. Therefore, if you are taking other medications, it is important to consult with your physician before starting this supplement. Specifically, this compound should not be taken alongside quinolone antibiotics, warfarin, or anti-hypertensive and anti-diabetic medications.

• Pregnancy and breastfeeding

There are not enough studies to support the safety of quercetin supplement use during pregnancy and breastfeeding.

• Kidney diseases

Quercetin is primarily excreted through kidneys.

• Although multiple academic studies suggest the beneficial effects of quercetin on human health, clinical outcomes do not support this conclusion, as its very poor oral bioavailability due to low absorption interferes with its activity in the human body.
• Quercetin taken orally before exercise does not reduce fatigue or enhance athletic performance.
• Quercetin is generally considered safe for most people when used short-term at the recommended doses. However, the safety of long-term use or higher doses is not well established.