Evening primrose oil (Oenothera biennis) is a commonly used alternative therapy and a rich source of omega-6 essential fatty acids. It is best known for its use in the treatment of systemic diseases marked by chronic inflammation, such as atopic dermatitis and rheumatoid arthritis. It is often used for several women’s health conditions, including breast pain (mastalgia), menopausal and premenstrual symptoms, cervical ripening, and labor induction or augmentation. However, there is insufficient evidence to make a reliable assessment of its effectiveness for most clinical indications. The current evidence suggests that oral evening primrose oil does not provide clinically significant improvement in persons with atopic dermatitis, and that it is also likely ineffective for the treatment of cyclical mastalgia and premenstrual syndrome. However, most trials to date have significant methodologic flaws and must be considered preliminary. The use of evening primrose oil during pregnancy is not supported in the literature and should be avoided. Evening primrose oil is generally well tolerated, with reported minor adverse effects, including gastrointestinal upset and headaches. Optimal dosing standards and treatment regimens await clarification in adequately powered clinical trials.
Bayles, Bryan and Usatine, Richard, "Evening Primrose Oil" (2009). Public Health. 3.