Oenothera biennis (common evening-primrose, evening star, sun drop, weedy evening primrose, German rampion, hog weed, King's cure-all, or fever-plant) is a species of Oenothera native to eastern and central North America, from Newfoundland west to Alberta, southeast to Florida, and southwest to Texas, and widely naturalised elsewhere in temperate and subtropical regions. Evening primrose oil (EPO) is produced from the plant.

The oil of mature seeds contains approximately 7–10% GLA. GLA, also referred to as C18:3-ω6 fatty acid, is not an essential fatty acid because it can readily be created from linoleic acid (C18:2-ω6). GLA is a preliminary stage in the production of prostaglandin, which is essential for the proper functioning of a cell. Symptoms and diseases like endogenous eczema, the Sjögren-syndrome, premenstrual syndrome (PMS), polyarthritis, multiple sclerosis and menopausal symptoms can be induced by a prostaglandin shortage. In theory, an additional intake of GLA might help to cure these symptoms.

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