The red-flowered hibiscus shrub is a widely cultivated ornamental, and because of its pleasant, tangy taste it is a common constituent of herbal beverage teas. Medicinally, hibiscus has been taken internally for the treatment of various forms of digestive upset, along with scurvy, anxiety, and fevers. It is said to have an antiseptic and astringent effect when used topically.

Animal studies have suggested that hibiscus might have a blood pressure–lowering effect.1,2 Based on this, preliminary human studies have been conducted. Unfortunately, all of these suffered from marked scientific flaws.

In one study, 54 people with hypertension were given either hibiscus tea or no extra treatment for 2 weeks.3 By the end of the study, people in the hibiscus group had significantly improved blood pressure as compared to those in the group receiving no extra treatment. Unfortunately, these results mean little; for a variety of reasons, people who are enrolled in a study and given a treatment tend to improve, regardless of whether the treatment itself actually works. In order to actually show that a treatment works, it must be compared against a placebo.

Another flawed study enrolled 90 people with hypertension and compared the effectiveness of hibiscus (10 grams dried hibiscus calyx in water daily) against the standard drug captopril (25 mg twice daily).4 The results showed equal benefit. Unfortunately, once more the study is less meaningful than it sounds. This study also failed to use a placebo group. In addition, it was not conducted in a double-blind manner. (For detailed information on why double-blind, placebo-controlled studies are essential, see Why Does This Database Rely on Double-blind Studies?)

In one double-blind study of hibiscus for hypertension, 171 people were given either hibiscus extract (250 mg anthocyaninsdaily) or the standard drug lisinopril (10 mg daily).16 The results showed that hibiscus was less effective than the standard drug. Properly speaking, this is the only conclusion that can be drawn from the study. The researchers take pains to emphasize that people taking hibiscus showed blood pressure improvements. However, as noted above, in the absence of a placebo to compare these improvements against, they cannot be taken as indicating any specific effect of hibiscus itself. Thus, at present, there is no reliable evidence that hibiscus exerts any anti-hypertensive effect.

In another double-blind trial, hibiscus was compared to black tea among 60 diabetic patients with mild hypertension.17 While subjects taking hibiscus significantly lowered their systolic (higher number) blood pressure over one month, those taking black tea significantly raised their systolic blood pressure over the same period. Again, without directly comparing hibiscus with a placebo, it is difficult to determine whether or not hibiscus effectively lowers blood pressure.

Finally, in a 2010 review of 4 trials involving 390 adults, investigators summarized the available research on Hibiscus sabdariffafor hypertension by concluded that there is insufficient evidence to support it effectiveness .18

Hibiscus contains substances called anthocyanins, antioxidants similar to those found in bilberry, cranberry, and red wine. Very weak evidence, too weak to be relied upon at all, hints that hibiscus or its anthocyanin constituents may haveanti-cancer5-10 and liver-protective11,12 effects, and might also improve cholesterolprofile.13,14