If you research quercetin side-effects, you’ll soon find that no clear toxicity exists. So taking large amounts won’t immediately have any obvious ill effects. However, there is some theoretical and anecdotal evidence that consuming large amounts of quercetin regularly — especially with other antioxidants (especially Vitamin C) — can potentially manifest some problems in some people despite quercetin benefits.

As is the case with many antioxidants — especially bioflavonoids such as quercetin — taking too many antioxidants at once can result in a pro-oxidant effect. What this means is that the inflammation-reducing effects of supplements such as vitamin c and quercetin can actually instead cause inflammation if you take them both in very high doses.

Anecdotal evidence suggests that this manifests itself most commonly in joint discomfort after prolonged usage of quercetin.