Onion bulb extract delivers a concentrated dose of onion, often standardized for the flavonoid quercetin and active sulfur compounds. Capsules and liquids are the usual forms, and most people take the supplement with food to ease digestion and boost absorption.
The extract helps maintain normal blood pressure and healthy cholesterol levels, and it supports immune function. Since it can thin the blood, do not combine it with anticoagulants like warfarin. The supplement may also lower blood pressure or blood sugar, so anyone on related medications or with those conditions should watch their numbers carefully. Stop using it two weeks before any scheduled surgery.