Dietary quercetin supplements: Assessment of online product informations and quantitation of quercetin in the products by high-performance liquid chromatography.

Administration of the increasingly popular dietary supplements containing quercetin may interfere with drug therapy. We intended to evaluate the online availability and quercetin content of the high-dose mono-component quercetin products and to review the potential use of quercetin products and their interactions with drugs. We monitored the online access to quercetin-containing dietary supplements, collected the relevant information from the websites, procured selected products from the vendors, and subjected them to substance analysis. The quercetin content was quantified by an HPLC-UV method. Twenty-five websites offered mono-component quercetin products, and nine products were procured. The quercetin content of eight products differed only ±10% from the nominal dose, whereas one product contained almost 30% more quercetin. Misleading indications such as antitumor and cardiovascular effects were often found on the sellers' websites. Quercetin-containing dietary supplements are available online with misleading indications. The recommended daily doses are often high (occasionally over 1,000 mg), which may induce clinically relevant interactions with medications. Because high-quercetin content of dietary supplements was confirmed, health care professionals should be aware of the unregulated internet market of dietary supplements and should consider the interactions of these substances with drugs.