It might be worth it for you to look up the terms Omega-6 fatty acid or gamma linolenic acid when searching for information on evening primrose:
Evening primrose is an Omega-6 fatty acid.You can look here for more information about Omega-6 fatty acids: http://www.alternativedr.com/conditions/ConsSupplements/Omega6FattyAcidscs.html
Evening primrose is categorized more specifically as Gamma linolenic acid. For more information about this, you can read the information included in the live link below.
One last note, it is cautioned that people who are on anti-convulsant medications and psychotherapeutic meds avoid this type of supplementation.