How you mean dangerous? I did do lots of research and understand it to be beneficial.
from mountsinai.org
“Very high doses of quercetin may damage the kidneys. You should take periodic breaks from taking quercetin. Pregnant women, breastfeeding women, and people with kidney disease should avoid quercetin. At doses greater than 1 g per day, there have been reports of damage to the kidneys.”
It is also a blood thinner.