Evening Primrose Oil for Symptoms of Premenstrual Syndrome and Menopause

By Dónal P. O'Mathùna, PhD. Dr. O'Mathùna is Senior Lecturer in Ethics, Decision- Making & Evidence, School of Nursing, Dublin City University, Ireland; he reports no financial relationships to this field of study.

A shift has occurred in recent years from viewing menopause as "a natural life event" experienced by women to "a condition that requires medical management."1 Following concerns about adverse events with hormone replacement therapy (HRT), many women have turned to complementary and alternative medicine (CAM), including herbs and dietary supplements. A survey in 2009 found that 45% of women who had discontinued HRT continued to use CAM, with evening primrose oil being one of the more common interventions used.1 Another survey found that 80% of women suffering from symptoms of premenstrual syndrome (PMS) use over-the-counter products, including herbs and natural remedies.2 Given that surveys report that up to 75% of women experience some PMS symptoms, clinicians should be aware of the evidence regarding this popular herbal remedy so they can provide patients with reliable information.3

Background

Evening primrose oil is obtained from the seeds of the yellow primrose (Oenothera biennis), a North American wildflower.4 Early English settlers brought the flower back to England where it was cultivated for its nut-flavored root. An oil was extracted from the seeds and became known as the King's Cure-All. Surveys have found that it remains very commonly used in England. Two evening primrose oil products were licensed by Britain's Medicines Control Agency. The products were available by prescription to treat atopic eczema and mastalgia (breast pain). However, in 2002 the licenses were revoked after a review of studies on its effectiveness found insufficient evidence to continue its official approval.5

Evening primrose oil contains a high proportion of essential fatty acids. The two most common types present in the oil are linoleic acid (about 65%) and gamma-linolenic acid (GLA, 8-10%), both omega-6 fatty acids.6 The oil also contains alpha-linolenic acid, an omega-3 fatty acid.7 Evening primrose oil is valued primarily for its GLA, being one of the richest plant sources. Only borage oil (24%) and black currant seed oil (16%) contain more GLA (see box). Unlike most omega-6 fatty acids, GLA is converted into a number of anti-inflammatory prostaglandins in the body, which is why evening primrose oil is often recommended to treat numerous chronic inflammatory diseases.

Mechanism of Action

How evening primrose oil might relieve premenstrual or menopausal symptoms is not clear. Some studies have shown that women with premenstrual syndrome tend to have lower than normal levels of GLA.6 Epidemiological studies have shown a connection between low dietary levels of GLA and a number of illnesses. However, a precise mechanism of action for evening primrose oil is not known.

Clinical Studies

Very few high-quality studies have been conducted using evening primrose oil for specific conditions in humans. A 1994 study remains the only controlled study of evening primrose oil for menopausal symptoms.8 The impact of evening primrose oil on hot flashes and night sweats was examined in 56 menopausal women randomized to either evening primrose oil (4 g/d plus 80 mg/d vitamin E) or placebo.9 Only 35 women finished the 6-month study. Although all women showed some improvements, no significant differences were found between the two groups.

Evening primrose oil has been one of the more popular natural therapies for PMS. Four randomized controlled trials have been identified on this topic,3,10 all conducted in the 1980s and 1990s. Three of the four were double-blinded, but all had small sample sizes (ranging from 22 to 38). The women took between 3 and 6 g evening primrose oil daily for between 4 and 10 months. All four studies failed to show any significant difference in overall PMS symptoms between evening primrose oil and placebo. One of the studies found that depression scores were significantly lower in the evening primrose oil group than the placebo group (P < 0.05).11 However, this was also the one study that was not double-blinded. This is an important limitation as other studies have found up to 80% of women report improved PMS symptoms with placebo.12

Evening primrose oil has been used to treat mastalgia, including that associated with PMS. One of the largest studies ever conducted on mastalgia randomized 555 women to one of four groups.5 Each woman took 4 g/d of capsules containing either evening primrose oil alone, evening primrose oil plus multivitamins, multivitamins alone, or placebo. After 4 months of blinded treatment, all groups showed an average 35% reduction in breast pain. No statistically significant differences were found between any of the four groups. The study continued as an open trial for another 6 months with all subjects receiving evening primrose oil, either with or without multivitamins. Breast pain was reduced by another 50%, but with no differences between the two groups. The researchers concluded that evening primrose oil was not superior to placebo in treating mastalgia. A meta-analysis of randomized controlled trials for mastalgia concluded that while bromocryptine, danazol, and tamoxifen use resulted in significant pain relief, evening primrose oil did not.13 This conclusion was based on the results of four controlled trials.

Adverse Effects

The most commonly reported adverse effects associated with evening primrose oil in clinical trials are difficulties swallowing the large capsules and gastrointestinal complaints. Most studies have used four 500 mg capsules taken twice daily, with up to 6 g/d used. The gastrointestinal symptoms are usually mild to moderate, with nausea being the most common. The effects of long-term use have not been examined. The withdrawal of evening primrose oil's license in Britain was not based on concerns about safety, but due to a lack of evidence of effectiveness.

As a side note, evening primrose oil has been popularly recommended to shorten the duration of labor, but a retrospective study found that women taking the oil during their pregnancies were in labor for an average of 3 hours longer.14 Although causation could not be established, women who are pregnant or breast feeding should, in general, avoid herbs and dietary supplements until they have been demonstrated to be safe.

Conclusion

A relatively small number of studies have examined the effectiveness of evening primrose oil in treating PMS and menopausal symptoms. The controlled studies consistently show that evening primrose oil is no more effective than placebo. Since these symptoms are known to respond well to the placebo effect, and to vary in random and cyclical ways, this may explain why the use of evening primrose oil remains popular.

In spite of its popularity, evening primrose oil is no more beneficial than placebo in treating menopausal or PMS symptoms. Some women may have very low levels of GLA in their diet, or may not produce adequate amounts within their bodies. They may receive some general health benefits from supplementing their diet with evening primrose oil. However, its usefulness in treating any particular condition associated with premenstrual syndrome or menopause is not supported by clinical research.

References

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11. Puolakka J, et al. Biochemical and clinical effect of treating the premenstrual syndrome with prostaglandin synthesis precursors. J Repro Med 1985;30:149-153.

12. Khoo SK, et al. Evening primrose oil and treatment of premenstrual syndrome. Med J Aust 1990;153:189-192.

13. Srivastava A, et al. Evidence-based management of mastalgia: A meta-analysis of randomised trials. Breast 2007;16:503-512.

14. Dove D, Johnson P. Oral evening primrose oil: Its effect on length of pregnancy and selected intrapartum outcomes in low-risk nulliparous women. J Nurse Midwifery 1999;44:320-324.