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Safety concerns about quercetin are considered in many studies. But the majority of them do not report any side effects. Still, it is recommended not to use excessive doses that may trigger unexpected adverse events. Studies have evaluated quercetin at a dose of 500 mg twice a day.
A total of 1000 mg of quercetin daily has no side effects when it is taken for 12 weeks. No study has evaluated this concentration for a longer time, and it appears to be very safe.
Long-term use and higher doses have not been thoroughly assessed. Therefore, we recommend not exceeding these doses in oral supplementation.
Among the side effects after oral administration of quercetin, we have the following:
- • Headache
- • Tingling in the arms and legs
Other studies have evaluated the effect of quercetin administered intravenously. These studies used doses of 722 mg and lower. Higher doses are not recommended. The side effects of intravenous administration of quercetin may include the following:
- • Facial flushing
- • Nausea and vomiting
- • Sweating
- • Difficulty to breath
- • Kidney damage with excessive doses
There is not enough information to evaluate the safety of quercetin for pregnant or breast-feeding mothers. Even though no adverse events are still reported, it is better to avoid using these supplements in such circumstances.
Since high doses of quercetin may cause kidney damage, it is recommended to stay away if you have kidney problems.
Also, if you’re taking other medications, consider that quercetin can interact with the following:
- Quinolone antibiotics: This group of antibiotics includes ciprofloxacin and levofloxacin. Quercetin may reduce the effectiveness of these antibiotics.
- Cyclosporin, amiodarone, verapamil: These medications are changed in the liver. Quercetin might interact with their metabolism. It increases the effects and may cause unexpected side effects when taken along with these medications.
- Other drugs: The list of medications changed by the liver is very large. Thus, ask your doctor if you use drugs for chronic disease. Other drugs that may interact with quercetin include medications moved by cell pumps—for example, itraconazole, digoxin, cimetidine, and ranitidine. *
