Efficacy of Hibiscus sabdariffa on reducing blood pressure in patients with mild-to-moderate hypertension: A systematic review and meta-analysis of published randomized controlled trials 

This 2022 meta-analysis and systematic review included data from 13 randomized control trials (1205 participants) relating to the efficacy of Hibiscus sabdariffa in patients with mild-to-moderate hypertension or metabolic syndrome. Authors concluded that Hibiscus sabdariffa is effective in reducing the SBP and DBP in patients with mild-to-moderate hypertension but was not more effective in those with MetS nor superior to antihypertensive drugs. 

References:

Abdelmonem, M., Ebada, M. A., Diab, S., Ahmed, M. M., Zaazouee, M. S., Essa, T. M., ElBaz, Z. S., Ghaith, H. S., Abdella, W. S., Ebada, M., & Negida, A. (2022). Efficacy of Hibiscus sabdariffa on Reducing Blood Pressure in Patients With Mild-to-Moderate Hypertension: A Systematic Review and Meta-Analysis of Published Randomized Controlled Trials. Journal of cardiovascular pharmacology, 79(1), e64–e74. https://doi.org/10.1097/FJC.0000000000001161