Evening primrose (Oenothera biennis, also known as willow herb, fever plant, or tree primrose) is native to North America east of the Rockies to the Atlantic coast. A biennial plant that grows wild in dry meadows and along roadsides, evening primrose has an erect, hairy stem that bears alternate, hairy leaves, which grow from 3 to 6 inches long and taper to a point. The plant flowers from June to October, bearing yellow, lemon-scented blossoms, 1 to 2 1/2 inches across, which open at dusk. The fruit of the plant is an oblong, hairy capsule, which contains small, reddish-brown seeds. Although the Native Americans and early European settlers used the whole plant for a variety of health applications, modern use has centered on the oil pressed from the seeds (EPO). This oil is primarily composed of linoleic acid, and omega-6 essential fatty acid found in most vegetable oils, but it also contains a significant amount of gamma linolenic acid (GLA), an omega-6 oil found in only a few plant seeds. Borage oil and black currant seed oil are also rich sources of GLA.
EPO supplements are commonly used by women seeking natural ways to alleviate the symptoms of PMS. A number of studies indicate EPO can improve many PMS symptoms such as depression, irritability, fluid retention, and breast pain. Although EPO's method of action in PMS is unclear, some scientists attribute its effects to increased PGE1 production, which may attenuate the action of some hormones such as prolactin.
Studies on EPO have used doses ranging from 2 to 4 grams per day, and it appears to be well tolerated at this dosage range. For PMS, the most commonly recommended dosage is 2 grams per day for 2 weeks preceding menstruation. Because other nutrients including magnesium, zinc, niacin, vitamin B-6, and vitamin C are needed along with EPO for the body to produce PGE1, some experts recommend taking EPO with a multivitamin that supplies these nutrients. EPO has no known toxicity.