People use evening primrose oil to treat several conditions, including rheumatoid arthritis; premenstrual syndrome, or PMS; eczema; and atopic dermatitis, says WebMD. Scientific evidence is inconclusive as to its effectiveness in treating PMS, and provides limited support for evening primrose oil's efficacy in treating eczema and rheumatoid arthritis.
Evening primrose oil has been used in traditional Native American medicine and Europe to treat hemorrhoids, bruises and wounds, says WebMD. It contains omega-6 fatty acids which are important to good health, but the impact of taking evening primrose oil as a supplement is not clear. People who take prescribed medications should be wary of taking evening primrose oil or any other supplement and should report all supplement usage to their doctor.