Background:
Quercetin, in general, belongs to the broader category of polyphenols. It is found, in particular, among the flavonols, along with kampferol, myricetin and isoramnetin and it is recognized as a foreign substance by the organism in contrast to vitamins. It occurs mainly linked to sugars with the most common compound being quercetin-3-O-glucoside, or as an aglycone, especially in the plant population. The aim of this review is to present the molecule of quercetin and the recent bibliography in the chemical field, on the mechanisms of absorption and metabolism, the bioavailability, the antioxidant and the clinical effect on diabetes, cancer and the coronavirus infection.

Methods:
Databases such as Scopus, PubMed, Science Direct and Google Scholar were used to collect the sources.

Results:
Recent literature reports a positive effect of quercetin on coronavirus patients, as well as it provides a protective effect against oxidative stress, cancer and helps with the regulation of blood sugar levels. Moreover, research-administered drug dosages of up to 2000mg per day showed mild to no symptoms of overdose. It should be noted, that quercetin is not considered a carcinogen substance. The daily intake of quercetin through the diet has been measured from 10mg-500mg, depending on the type of products consumed.

Conclusions:
Due to the lack of recent literature, a specific daily energy need for this substance has not been determined. Therefore, further studies must take place in order for the right administered quantity of quercetin to be considered beneficial to the body.