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Quercetin is a naturally occurring flavonoid, present in a range of fruit and vegetables. However, dietary consumption of quercetin results in a relatively low dosage of up to 40 mg/day.
Benefits and Effects
May improve exercise performance
A consolidation of various studies using quercetin to improve exercise performance through measures of VO(2max) and endurance exercise performance shows a small improvement. 
Plasma quercetin levels were measured in a group of cyclists twice as high as taking quercetin alone. Moderate reductions in inflammation, oxidative stress and immune disruption were recorded.
May lower stroke risk
Researchers investigated the dietary causes of strokes involving 20,000 people. Over 10 years, 233 of these participants suffered from strokes. Researchers found that having white fruit and vegetables, containing Quercetin, in the diet lowers the risk of stroke by 52%. 
Reduces prostate pain
Prostatitis is a condition where the prostate gland becomes inflamed and swollen, leading to muscle pain, and lower back pain.  Quercetin has been tested as a treatment for prostatitis with very promising results. Some studies show as high as 82% success rates for reduction in symptoms for prostatitis patients. 
Improves sex drive
Quercetin is a phosphodiesterase (PDE) inhibitor. PDE4 inhibitors have been tested on mice to determine the effect on libido.  Researchers investigating quercetin as a PDE4 inhibitor have found it to have higher PDE4 ratios than other candidates already progressing through clinical trials. 
How the Quercetin Works
Quercetin is absorbed through the intestine. After quercetin glycosides are hydrolyzed, the released quercetin aglycone permeates through the intestinal epithelial barrier; consequently, metabolized in the enterocytes and liver. 
General bioavailability of quercetin is moderate, and has a low neural bioavailability.  As far as its action in the testicles is concerned, quercetin prevents testicular damage from dioxins. 
Quercetin binds DNA. This binding causes conformational changes to DNA. Quercetin treatment seems to activate apoptosis in tumor tissues due to its action on the DNA. 
A normal dietary intake would be between 5 and 40 mg/day. Common quercetin supplement dosage would however be much larger: 1,000 mg/day in 2 doses of 500 mg each.  An effective quercetin dose has not been properly established.
Make minor changes to supplement dosages in order to monitor any adverse side effects.
Adverse effects were rare, as reported by participants and that those reported were mild in nature. One patient reported headaches after the first few quercetin doses. Some patients have also experienced nausea when taking quercetin on an empty stomach. 
It is not advisable to take quercetin with antibiotics. Quercetin binds to the bacterial DNA gyrase. Cyclosporine, Estradiol and Felodipine are other drugs that may be affected by quercetin.  It is interesting to note the quercetin enhances the bioavailability of some drugs while inhibiting the bioavailability of others.