Evening Primrose Oil (EPO) is derived from the seeds of the evening primrose plant (Oenthera biennis). It is native to North America. The irony is that there it is regarded as a weed. This biennial plant is thought to be a complex of several closely related species belonging to the Onagraceae family.
EPO is a rich source of the long-chain fatty acid gamma-linolenic acid (GLA). The health benefits of EPO are attributed to GLA. GLA is an unusual constituent of living matter and is found in very few plants. Typical EPO supplements contain about 9% GLA.