What is evening primrose oil?

Evening primrose (Oenothera biennis) is a traditional medicinal herb native to North America that now grows in many places worldwide. The plant is suitably named for its yellow flowers that bloom in the evening after sunset.^[1] Evening primrose oil (EPO) is extracted from the seeds of the evening primrose plant and is a rich source of omega-6 fatty acids, particularly linoleic acid (70–74%) and gamma-linolenic acid (GLA) (8–10%).^[2] While the fatty acid profile of EPO is generally thought to be the main factor driving its effects in the body, EPO also contains other bioactive compounds, including triterpenes, phenolic acids, and tocopherols (vitamin E).^[2] Despite being promoted as a remedy for a variety of ailments, most clinical research has not identified any clear benefits of EPO supplementation.