The current evidence suggests that oral evening primrose oil does not provide clinically significant improvement in persons with atopic dermatitis, and that it is also likely ineffective for the treatment of cyclical mastalgia and premenstrual syndrome. However, most trials to date have significant methodologic flaws and must be considered preliminary. The use of evening primrose oil during pregnancy is not supported in the literature and should be avoided. Evening primrose oil is generally well tolerated, with reported minor adverse effects, including gastrointestinal upset and headaches. Optimal dosing standards and treatment regimens await clarification in adequately powered clinical trials.
– Am Fam Physician. 2009 Dec 15;80(12):1405-8. — Evening primrose oil. Bayles B, Usatine R.

While recommending this supplement to patients for many years for the treatment of mastalgia, this author was interested in researching the plant and the evidence supporting its use for this complaint and its other potential uses. The oil is rich in omega-6 fatty acids, which are essential for many bodily functions; however, a lack of strong scientific evidence exists to support its use for the relief of mastalgia or atopic dermatitis.
– Holist Nurs Pract. 2008 May-Jun;22(3):171-4. — A review of the clinical efficacy of evening primrose.
Stonemetz D.

There is mixed evidence for evening primrose and borage lowering the seizure threshold.
– 2001 Dec;2(6):524-532 — Herbal Medicines and Epilepsy: The Potential for Benefit and Adverse Effects. — Spinella M.

Use caution if pregnant or lactating.

It may cause headaches or nausea if taken on an empty stomach, and diarrhea if taken in high doses.

Evening Primrose 0il should not be used by patients with epilepsy or schizophrenia.