Quercetin is generally well tolerated and appears to be associated with little toxicity when administered orally or intravenously. Adverse effects may include nausea, dyspnoea, headache and mild tingling of the extremities.
Possible modulation of P-glycoprotein and inhibition of CYP1A1, CYP1A2 and CYP 3A4 activity should be considered when prescribing.
Quercetin pretreatment may sensitise human cervix carcinoma cells to cisplatin-induced apoptosis — beneficial interaction theoretically possible under professional supervision.
Animal studies demonstrate that coadministration of quercetin significantly decreases the oral bioavailability of cyclosporin — avoid concurrent use.
An increase in drug bioavailability is theoretically possible and has been observed in an in vivo study. Although human studies at lower doses are not available, the narrow therapeutic range of digoxin and the serious nature of the interaction should not be underestimated. Avoid concurrent use.
Pretreatment of rabbits with quercetin resulted in an increased bioavailability of the calcium channel blocker, diltiazem, which may be the result of inhibition of P-glycoprotein and CYP 3A4. Caution — use under professional supervision; doses may need to be adjusted accordingly
Tardive dyskinesia (rhythmical involuntary movements of the tongue, face, mouth or jaw e.g. protrusion of tongue, puffing of cheeks, puckering of mouth, chewing movements) may result from long-term therapy with the antipsychotic medication haloperidol and may be irreversible in some individuals with no known effective treatment. Oxidative stress and the products of lipid peroxidation have been implicated in the pathophysiology of tardive dyskinesia and co-administration of quercetin (25-100 mg/kg) has been shown to dose-dependently reduce haloperidol-induced vacuous chewing movements and tongue protrusions in animal models. Beneficial interaction theoretically possible under professional supervision.
Pretreatment with quercetin may increase the bioavailability of paclitaxel according to animal studies. Caution — use under professional supervision; doses may need to be adjusted accordingly.
According to animal data, pretreatment with quercetin may reduce the risk of mortality from paracetamol overdose. However, effects in humans have not been studied — beneficial interaction theoretically possible.
In vitro, quercetin binds to the DNA gyrase site in bacteria and therefore may theoretically compete with quinolone antibiotics that also bind to this site (PDRHealth 2005) — caution.
Concurrent use of quercetin with the antileishmanial drug stibanate appears to improve the efficacy of the drug and reduce the anaemia and parasitaemia associated with the condition — beneficial interaction theoretically possible.
Contraindications and Precautions
Hypersensitivity to quercetin.
Safety in pregnancy has not been established.