According to epidemiological research, the antioxidant flavonol quercetin, which can be found in apples, berries, and onions, is linked to a lower risk of coronary heart disease and stroke. Additionally, supplementing with quercetin lowers blood pressure in hypertensive rodents. It has never been tested whether quercetin supplementation can effectively lower blood pressure in hypertensive people. We looked into the claim that quercetin supplementation lowers blood pressure in people with hypertension. Next, we explored whether quercetin’s antihypertensive effect is linked to decreases in systemic oxidative stress. A randomised, double-blind, placebo-controlled, crossover trial was conducted on men and women with prehypertension (n = 19) and stage 1 hypertension (n = 22) to compare the effectiveness of 730 mg of quercetin per day for 28 days vs a placebo. At enrolment, prehypertensives’ blood pressure (mm Hg, systolic/diastolic) was 137 +/- 2/86 +/- 1 and stage 1 hypertensive individuals’ blood pressure was 148 +/- 2/96 +/- 1. After quercetin administration, blood pressure in prehypertensive patients did not change. On the other hand, stage 1 hypertension patients who received quercetin medication saw decreases in their systolic (-7 +/- 2 mm Hg), diastolic (-5 +/- 2 mm Hg), and mean arterial pressures (-5 +/- 2 mm Hg) pressures (P 0.01). Quercetin had no effect on the oxidative stress indicators seen in the plasma and urine, though. To our knowledge, these are the first evidence demonstrating quercetin supplementation lowers blood pressure in hypertensive patients. Contrary to research using animals, there was no systemic oxidative stress marker reduction induced by quercetin.