The study was designed to compare the blood pressure lowering effects of sour tea (ST) -Hibiscus sabdariffa with black tea (BT) in type II diabetics with mildly high blood pressure.
Patients were randomly assigned to drink one cup of Hibiscus or black tea two times per day for one month. Each infusion contained one tea sachet weighing 2 gm, placed in a tea pot with 240 ml boiling water and steeped for 20-30 minutes and then added one cube of sugar.
The average systolic blood pressure (SBP) in the hibiscus group decreased from 134.4 +/- 11.8 mm Hg at the start of the study to 112.7 +/- 5.7 mm Hg after 1 month. The average SBP changed from 118.6 +/-14.9 to 127.3 +/- 8.7 mm Hg in the black tea group during the same time period. There were no statistically significant effects on the mean diastolic blood pressure in either group.
Commentary: 
It’s always gratifying to see a simple, safe, inexpensive herb studied for such a common problem. Hibiscus has been used historically for high blood pressure and contains several important ingredients including alkaloids, anthocyanins and quercetin. It is thought that the antioxidant and diuretic effects are the most important mechanisms.
References
Mozaffari-Khosravi H, Jalali-Khanabadi B, Afkhami-Ardekani M, et al. The effects of sour tea (Hibiscus sabdariffa) on hypertension in patients with type II diabetes. J Human Hypertension 2009;23:48-54.
