Research has not shown that evening primrose oil is effective in treating premenstrual syndrome, breast pain or hot flashes, according to WebMD. Side effects include problems with blood clotting and the immune system, headache, nausea and diarrhea. The botanical may also interact negatively with other medications and conditions.
Evening primrose oil may increase the risk of bleeding in individuals taking anticoagulant and antiplatelet medications, cautions WebMD. Individuals taking phenothiazines, a class of drugs used to treat schizophrenia and other psychotic disorders, may experience serious nausea, vomiting and seizures when using evening primrose oil. Evening primrose oil may also interact with certain depression medications. Individuals with epilepsy and bleeding disorders should be especially cautious with the supplement, and people taking blood pressure medications should be aware that combining them with evening primrose oil might lower blood pressure even further. The supplement may cause seizures when combined with anesthesia in individuals undergoing surgery.
A 2013 study published by the National Institutes of Health cited by Healthline shows that evening primrose oil can be an effective eczema treatment for children and teens at doses of 160 milligrams and 360 milligrams. Some research shows that evening primrose oil may be helpful in treating atopic dermatitis and rheumatoid arthritis, but more research is needed, states WebMD.