Randomised controlled trials (RCTs) that compared hibiscus against placebo, another herbal or pharmacological intervention, or no intervention, in the treatment of primary hypertension in adults (18 to 70 years) who had a diagnosis of pre-hypertension (130 to 139 over 85 to 89) or primary hypertension (stage I or II of the Joint National Committee, JNC 7, classification), were eligible for inclusion. The primary outcome was the mean reduction in systolic or diastolic blood pressure or both.
Hibiscus was administered as hibiscus extract or sour tea; the quantity and method of preparation of hibiscus varied between trials. Comparator interventions were black tea, captopril, and lisinopril. Participants in the included trials had pre-hypertension, stage I hypertension, or stage II hypertension. Study duration ranged from two to four weeks.
Two reviewers independently assessed trials for inclusion and disagreements were resolved through discussion.