Onion supplements usually come as dried bulb extract or powder. They provide sulfur compounds like isoalliin and the flavonoid antioxidant quercetin. These compounds differ from garlic’s, though they share some properties.
Early research shows onion might help maintain healthy blood pressure, cholesterol, and platelet function. However, most evidence is from animal or lab studies, and human trials are scarce with mixed results. Onion also supplies prebiotic fibers that may support digestive health.
The dried extract is taken at 100 to 1000 mg daily, or as a liquid tincture, typically with meals to reduce stomach upset. Quercetin absorption may improve when taken with fat-containing food.