Fennel

Fennel: In several randomized, placebo controlled trials, fennel seed was found to be as effective as NSAIDs for treating menstrual pain — all without the potential side effects.

• A 2012 study found that fennel seed was more effective than a placebo for controlling severe menstrual pain. On average, women started the study with 6/10 menstrual pain, and five hours after taking fennel seed rated their pain just a 1/10. 
• Fennel seed isn't just better for cramps than placebo pills, it held it's own against a potent pharmaceutical pain killer (similar to ibuprofen, but requiring a prescription). In a study of severe menstrual pain in high school students, there was no statistically significant difference between fennel seed and mefenamic acid, a preferred NSAID for treating menstrual pain. However, there were no reports of side effects in the fennel seed group, while possible side effects of mefenamic acid include nausea, diarrhea, rashes, autoimmune anemia, and kidney toxicity. A later study confirmed these results. 
• Fennel seed has also been shown to relieve nausea and fatigue associated with menstrual bleeding, decrease the duration of the period, and improve overall feelings of well-being when taken over several cycles.