Evening Primrose Oil: Benefits, Evidence and Side Effects

Evening primrose oil (EPO) is pressed from the seeds of Oenothera biennis and is sold mainly for its omega-6 fat, gamma-linolenic acid (GLA). It is marketed for premenstrual symptoms, breast pain, menopause and skin, but the National Center for Complementary and Integrative Health (NCCIH) concludes there is not enough evidence to support it for any health condition. The largest body of research, on eczema, found it worked no better than a placebo. EPO is generally well tolerated, though it can upset the stomach, may add to bleeding risk with blood thinners and should not be used to try to start labor without medical guidance. Here is what the studies found, the side effects and what to check first.

Peptide Labs does not sell evening primrose oil, so no product is recommended here. This article is general information, not medical advice.

What is evening primrose oil?

Evening primrose is native to North and South America and now grows in Europe and parts of Asia. Its yellow flowers open at sunset and close during the day, which gives the plant its name. NCCIH notes that Native Americans applied juice from the stem and leaves to the skin for bruises and minor wounds and used the leaves for digestive complaints and sore throats. Today the seed oil is sold in softgels and skin products and is promoted for eczema, rheumatoid arthritis, PMS, breast pain and menopause.

The oil’s selling point is gamma linolenic acid, an omega-6 fatty acid that MSKCC says makes up about 10% of EPO. Borage oil also contains GLA. Despite the similar letters, EPO is not the same as EPA, one of the omega-3 fats in fish oil. If omega-3s are what you are after, see our omega-3 guide and our comparison of krill oil vs fish oil. Black seed oil is a different plant oil again.

Evening primrose oil benefits: what the evidence says

EPO has been studied for decades, and the overall picture is disappointing. NCCIH’s summary is blunt: oral EPO has not been shown to help atopic dermatitis, it is probably no better than placebo for breast pain, studies on starting labor are inconsistent and there is not enough evidence for other uses.

Eczema

A 2013 Cochrane review pooled 27 randomized trials with 1,596 participants, 19 of them testing EPO and 8 testing borage oil. Compared with placebo, EPO did not significantly improve overall eczema symptoms as rated by either participants or doctors, and most of the studies had a low risk of bias. Because the results ruled out any useful difference so narrowly, the reviewers said further trials would be hard to justify. One author disclosed that his earlier research was funded by an EPO maker. Eczema that itches, cracks or keeps flaring is worth a visit to a dermatologist.

Breast pain

In a 2002 trial, 120 women with severe, long-lasting breast pain took EPO, fish oil, both or control oils for six months. Days with pain fell by 12.3% on EPO and 13.8% on its control oil, so EPO offered no clear benefit. A smaller 2010 pilot study enrolled 85 women but only 41 finished, and its comparisons with placebo were not statistically significant. New, one-sided or persistent breast pain, or any lump, should be checked by a clinician.

Premenstrual symptoms

A 1996 review found only seven placebo-controlled trials of EPO for PMS, and randomization was clearly described in just five. Scoring varied so much that the results could not be pooled, and the two most tightly controlled trials found no benefit. The trials were small, so a modest effect cannot be ruled out, but NCCIH still rates the evidence as insufficient. Our vitex guide covers chasteberry, the herb with more (if low-quality) premenstrual research.

Menopause

A six-week trial in Iran randomized 56 women aged 45 to 59 to EPO or placebo. Hot flash severity improved more with EPO, but changes in how often flashes happened and how long they lasted were not significantly different from placebo, which also improved. A single small, short trial is not enough to draw conclusions. Clinicians can discuss other options for bothersome hot flashes.

Pregnancy and labor

Some people use EPO by mouth or vaginally late in pregnancy in hopes of starting labor, on the theory that it raises prostaglandins. NCCIH says the results are inconsistent and long-term safety is not established. MSKCC cites a study in which women who used EPO to shorten pregnancy had more prolonged rupture of membranes, oxytocin use, stalled labor and vacuum deliveries, and it advises against EPO during pregnancy. Sources differ on how cautious to be, so use it in pregnancy only if your obstetric clinician agrees.

Evening primrose oil side effects

NCCIH says EPO is probably safe for most adults when taken by mouth, though less is known about children. The most common evening primrose oil side effects are stomach pain, nausea and diarrhea; MSKCC adds indigestion, softer stools and headaches. In the Cochrane trials, side effects were mild, short-lived, mainly digestive and similar to placebo, but those studies were short and did not look at long-term use. MSKCC also notes a case of lipoid pneumonia, a lung problem caused by inhaling oil particles, after long-term use.

Interactions and who should ask first

• Blood thinners and antiplatelet drugs: EPO may add to bleeding risk. In one small study, bleeding time rose in 9 of 12 people after months of GLA from EPO, and the Cochrane review cites a report of more bleeding in people on warfarin. Our nattokinase guide covers another supplement with the same concern.
• Blood pressure medicines: a large population study identified EPO among supplements linked with higher blood pressure, so tell your clinician if you are monitoring it.
• HIV medicines: MSKCC notes EPO may raise levels of the antiretroviral lopinavir.
• Pregnancy and breastfeeding: evidence is not conclusive. NCCIH notes EPO raises GLA in breast milk without linked harm to infants, while MSKCC advises against use in pregnancy.
• Seizure disorders: older warnings came from two reports in the early 1980s, and a 2007 re-examination judged the link spurious. Still, talk with your neurologist before adding any supplement.
• Hormone-sensitive cancers: EPO itself is not estrogenic, but MSKCC warns that some products combine it with plant estrogens, so read the full label.
• Before surgery: tell your care team about every supplement you take, including EPO.

How to choose an evening primrose oil supplement

1. Confirm the source: the label should name Oenothera biennis seed oil.
2. Check whether GLA content is stated. Products vary, and labels that list it are easier to compare.
3. Read the full ingredient list. Formulas for women may add plant estrogens or other herbs.
4. Look for an independent quality seal. Our guide to third-party tested supplements explains what those seals cover.
5. Follow the label’s storage directions and check the expiration date, since it is an oil.
6. Read claims about skin, hormones or PMS as marketing. The FDA does not approve supplements or their claims before sale.

Evening primrose oil vs vitex

The two are often sold side by side but are unrelated. Vitex is a fruit extract thought to act on hormone signals from the brain, and its premenstrual trials lean positive but are low quality. EPO is a seed oil rich in omega-6 fat, and its trials mostly show no clear benefit over placebo. Neither is FDA-approved for any condition, and both carry cautions in pregnancy. If symptoms affect your daily life, a clinician is a better first stop than either bottle.

Frequently Asked Questions

Questions about a label? Contact us or read our Buyer’s Guide. Follow the label and ask a healthcare provider before starting any supplement, especially if you take medicines. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Last reviewed October 2026.

Related guides: Vitex (Chasteberry): What the Research Shows and Who Should Avoid It, Omega-3: Benefits, Foods, Side Effects and Heart Research, Black Seed Oil: Benefits, Research, Side Effects and Who Should Avoid It.