Oenothera is a taxonomically difficult genus of about 120 species, originating in the New World and introduced to Eurasia. Hybridization and morphological variability due to combinations of unusual ?ring? chromosomes are frequent. Evening primrose, itself quite variable, is widely distributed in North America and Europe, and is cultivated for its large, attractive yellow flowers, which open in the evening. Native American tradition used the roots medicinally, and Europeans used the stem ?bark? and leaves. In recent decades, evening primrose seed oil (EPO) has become popular to treat premenstrual syndrome (PMS) and other conditions, such as atopic eczema and asthma. It contains beneficial fatty acids that are scarce in the modern diet, and that affect prostaglandin metabolism. EPO combined with fish oil has been used as a dietary supplement for pregnant women and nursing mothers and to treat dogs suffering from allergic dermatitis.

There have been quite a few human clinical trials of oral and topical EPO preparations to treat premenstrual breast pain and skin conditions. Almost all have shown little or no significant benefit. Possible flaws in some of these studies include inadequate dosage, use of a placebo oil that also affects plasma fatty acids, and use of an inappropriate formulation in topical preparations. However, the balance of the evidence suggests that EPO is probably not very effective, so that reported benefits may be due more to ?placebo effects? than to specific bioactivities of the oil. A few studies have shown modest benefit for other conditions (ulcerative colitis, uremic skin symptoms in dialysis patients, prevention of pre-eclampsia), but further confirmation of those results is needed.

Selected References

Blommers, J., E. S. de Lange-De Klerk, D. J. Kuik, P. D. Bezemer, and S. Meijer. 2002. Evening primrose oil and fish oil for severe chronic mastalgia: a randomized, double-blind, controlled trial. Am. J. Obstet. Gynecol. 187:1389-1394.

Dietrich, W., W. L. Wagner, and P. H. Raven. 1997. Systematics of Oenothera section Oenothera subsection Oenothera (Onagraceae). Syst. Bot. Monogr. 50:1-234.

Gehring, W., R. Bopp, F. Rippke, and M. Gloor. 1999. Effect of topically applied evening primrose oil on epidermal barrier function in atopic dermatitis as a function of vehicle. Arzneimittelforschung 49:635-642.

Steiner, E., and D. A. Levin. 1977. Allozyme, SI gene, cytological, and morphological polymorphisms in a population of Oenothera biennis. Evolution 31:127-133.

Yoshimoto-Furuie, K., K. Yoshimoto, T. Tanaka, S. Saima, Y. Kikuchi, J. Shay, D. F. Horrobin, and H. Echizen. 1999. Effects of oral supplementation with evening primrose oil for six weeks on plasma essential fatty acids and uremic skin symptoms in hemodialysis patients. Nephron 81:151-159.