Oil extracted from seeds of the evening primrose (Oenothera biennis) contains linoleic acid, (gamma) linolenic acid, and vitamin E. (gamma) Linolenic acid is a precursor of prostaglandin E and several other active substances and is said to be the constituent of the oil responsible for its therapeutic effects. Disorders for which evening primrose oil has been tested in controlled clinical trials include atopic dermatitis, rheumatoid arthritis, diabetic neuropathy, multiple sclerosis, various cancers, Raynaud's phenomenon, ulcerative colitis, pre-eclampsia, the pre-menstrual syndrome, menopausal flushing, breast cysts, mastalgia, Sjogren's syndrome, schizophrenia, and hyperactivity.1 What are the results of these clinical trials?

Many of the studies have been crossover trials, which is a pity for two reasons. Firstly, crossover trials are really suitable only for assessing drugs whose effects fade rapidly after treatment has been stopped. Any ersistent effects will disappear provided there is a “washout” period before the crossover. Secondly, if the explanation given to patients before their informed consent is obtained includes the timing of the crossover their expectations may become a major source of bias. The treatment, the natural course of the disease, and …