Fennel contains anethole, which can explain some of its medical effects; it, or its polymers, act as phytoestrogens. The essence of fennel can be used as a safe and effective herbal drug for primary dysmenorrhea, but could have lower potency than mefenamic acid at the current study level. Fennel is widely employed as a carminative, both in humans and in veterinary medicine (e.g., dogs), to treat flatulence by encouraging the expulsion of intestinal gas. Anethole is responsible for the carminative action.