Derived from the plant Oenothera biennis, evening primrose oil is used for rheumatoid arthritis, premenstrual syndrome, eczema, fatigue, diabetic neuropathy, and mastalgia. The mucilaginous stem and leaf juices have been used as a poultice to treat minor bruises and wounds, and to soothe skin inflammation ⁽²⁵⁾. Evening primrose oil is thought to improve skin moisture and to reduce transepidermal water loss ⁽²⁶⁾. It is also among the popular natural products used to relieve menopausal symptoms ^(27) (28). In vitro, evening primrose oil demonstrates anti-inflammatory activity ⁽²⁹⁾ and inhibits platelet aggregation ^(6) (7). In animal models, it exerts anti-angiogenic, anti-inflammatory, and anti-arthritic effects ⁽³⁰⁾ and improved cardiac recovery after myocardial infarction ⁽³¹⁾.

Small clinical studies suggest its effectiveness against atopic dermatitis ^{(18) (25) (32)}, but other analyses have concluded that neither evening primrose oil nor borage oil are effective for eczema ^(23) (33). A small study in acne patients treated with oral isotretinoin suggests that evening primrose oil can improve xerotic cheilitis ⁽³⁴⁾, and may be effective against 5-azacitidine-induced skin reactions in patients with myelodysplastic syndrome (MDS) ⁽¹⁹⁾. It was also found useful in preventing weight regain following extensive weight loss ⁽⁵⁾. A meta-analysis suggests it may relieve rheumatoid arthritis symptoms ⁽²¹⁾. Other studies show modest benefit in patients with ulcerative colitis ^(35) (36) or in ocular surface diseases such as dry eye ^(37) (38). Results from previous studies that evaluated possible benefits for diabetic neuropathy patients were equivocal ⁽³⁹⁾. But several studies suggest that co-supplementation with hemp seed and evening primrose oils along with a diet high in total antioxidant capacity may improve clinical and immunological parameters in patients with multiple sclerosis ^{(40) (41) (42)}.

Additional data indicate that supplementation with vitamin E and evening primrose oil reduced cyclical mastalgia ⁽²⁰⁾, but other analyses did not find improvements in breast pain ^(1) (43) or premenstrual syndrome ⁽²²⁾. Evening primrose oil may improve quality of life and decrease hot flash intensity in menopausal women ⁽⁴⁴⁾, although other data suggest that behavioral/lifestyle approaches such as exercise provide better relief ⁽²⁸⁾. It has not been studied extensively for cancer, but a study involving liver cancer patients showed no effect on tumor size or survival ⁽⁴⁵⁾. Another small study suggested that gamma-linolenic acid (GLA) may be an effective adjunctive therapy for breast cancer ⁽⁴⁾.

Although evening primrose oil does not have intrinsic estrogenic properties, some commercial products combine evening primrose oil with phytoestrogens. Therefore, patients with hormone-sensitive cancers should use evening primrose oil products with caution.