Fennel for Reducing Pain in Primary Dysmenorrhea: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Fennel is used as an alternative treatment for primary dysmenorrhea. This review aims to evaluate the effectiveness and safety of fennel for reducing pain in primary dysmenorrhea. Twenty databases, including English, Korean, Chinese, Japanese, Iranian, and Spanish databases, were searched from inception to 20 October 2020. All randomized controlled trials (RCTs) investigating the effectiveness of fennel for treating primary dysmenorrhea were considered. Two reviewers conducted the data extraction and risk of bias assessment independently. Any discrepancies were resolved through discussion with a third reviewer. A total of 12 studies were included in this review. The pooled results of seven trials showed that the effect of fennel is similar to that of conventional drug therapies in alleviating pain (n = 502, standardized mean difference (SMD): 0.07, 95% confidence interval(CI): -0.08 to 0.21, p < 0.37, I-2 = 0%). In comparison with placebo, fennel was seen to have favorable effects on reducing pain in primary dysmenorrhea (n = 468, SMD: -3.27, 95% CI: -5.28 to -1.26, p = 0.001, I-2 = 98%). Only three studies assessed adverse events (AEs), and one study reported minor AEs. Although the risk of bias for all the included studies was moderate, potential publication bias was observed due to the presence of a greater number of small studies with favorable effects. This systematic review concludes that fennel is as effective as conventional drug therapies in alleviating pain in primary dysmenorrhea. More studies that include more diverse populations and robust evidence of fennel's effects will be needed in future research endeavors.