Evening primrose oil: what it is and whether it helps PMS

Evening primrose oil comes from the seeds of Oenothera biennis, a pale yellow wildflower native to North America. The oil is high in two omega-6 fatty acids, linoleic acid and gamma-linolenic acid (GLA), with GLA making up roughly 8 to 14 percent of the total oil (). GLA is a building block for prostaglandins, hormone-like compounds involved in inflammation and pain.

That's the theory behind most of its uses: more GLA, in theory, means less inflammatory signaling. In practice, evening primrose oil is sold as a dietary supplement, not a medicine. It isn't held to the same testing or purity standards as a prescription drug, so the amount of GLA and the overall quality can vary between brands. Capsules are the most common form, usually taken with food.

Most research on evening primrose oil looks at inflammatory and hormone-related conditions rather than a single use. A 2024 systematic review of clinical trials found some supporting evidence for diabetes, atopic eczema, menopausal hot flashes and cyclical breast pain (mastalgia), but no benefit for premenstrual syndrome, acne, psoriasis or several other conditions it's been tried for ().

The best-studied women's health use is breast pain. In a randomized, placebo-controlled trial, the group taking 3,000 mg a day of evening primrose oil for six months saw its worst-pain score improve significantly from baseline (p=0.005), though the direct comparison with the placebo group didn't reach significance (p=0.18) (). It's also been tried for rheumatoid arthritis and skin hydration, with mixed or modest results.

So evening primrose oil may ease certain inflammation-linked pain and some skin or breast symptoms for some people, but for several conditions it's marketed for, including PMS, well-designed trials haven't shown it beats placebo.

The evidence here is thin but slightly more encouraging than for PMS. A 2024 systematic review found some supportive results for menopausal hot flashes among the inflammatory conditions it covered (), and a separate review of evening primrose oil in women's health also lists hot flashes among the conditions studied ().

Neither source treats evening primrose oil as a reliable fix for hot flashes, and larger, rigorous trials on hot flash frequency and severity are limited. Most of what's known comes from broader reviews rather than trials built specifically around hot flashes, so treat any improvement as a possible side benefit, not a guarantee. If hot flashes are disrupting your sleep, some prescription options have stronger trial evidence behind them than evening primrose oil does.

For PMS, the trial evidence leans negative. A systematic review found only seven placebo-controlled trials, with clear randomization in five of them, and the two best-designed showed no benefit for evening primrose oil over placebo (). A crossover trial of 38 women found PMS symptoms improved over six cycles, but just as much on placebo as on the oil, a placebo effect rather than a drug effect (). A smaller trial in women with PMS-related anxiety and low mood found no real difference between the oil and placebo either ().

Newer studies combining the oil with vitamin B6 or tocotrienol report large symptom drops (,), but they can't isolate what evening primrose oil alone is doing. See evening primrose oil for PMS for the trial-by-trial detail.

PMDD requires at least five symptoms, including a mood symptom, confirmed by daily tracking over two cycles (DSM-5-TR). No trial above enrolled women with a confirmed PMDD diagnosis, so there's no direct evidence either way; see evening primrose oil for PMDD.

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There's no official dose, since evening primrose oil isn't an approved medicine, but trials give a sense of what's been tested. PMS trials have used 1,000 mg capsules taken twice a day (2,000 mg total) for two menstrual cycles (). The mastalgia trial that found a benefit used 3,000 mg a day for six months ().

A review of evening primrose oil in women's health notes that you shouldn't expect an immediate response, and suggests using it regularly for four to six months before judging the effect (). That timeline matters. Trying it for two weeks and stopping isn't a fair test, though the PMS trials that ran for months still didn't find a benefit over placebo.

If you try it, check the label for GLA content, since that's the active ingredient, not just total oil weight, and ask a pharmacist about a dose that fits you. For more on timing, see evening primrose oil dosage.

Evening primrose oil is generally well tolerated. The side effects described most often are mild stomach upset, nausea, loose stools and headache, and taking the capsule with food can ease these.

Because GLA affects prostaglandins involved in blood clotting, evening primrose oil may have a mild blood-thinning effect. That's one reason it's grouped with other fatty-acid supplements that call for caution before surgery or alongside blood-thinning medication (more on that below).

If you develop a rash, unusual bruising or any bleeding that doesn't make sense after starting evening primrose oil, stop taking it and tell your doctor. For more on what's been reported, including effects on mood, see evening primrose oil side effects, including mood.

Because of its possible effect on blood clotting, evening primrose oil is generally advised against for people on blood thinners (anticoagulants or antiplatelet drugs), since combining the two could raise bleeding risk. The same caution applies before any planned surgery.

It may also interact with epilepsy medications or other drugs that affect the brain or mood; trial data on these interactions is limited. None of the clinical trials cited here, including the PMS trials, were designed to test drug interactions, so this caution comes from pharmacology, not dedicated trials.

If you take any prescription medication, including hormonal birth control, antidepressants or blood thinners, check with a pharmacist before adding evening primrose oil. For a wider look at what actually helps with PMS day to day, see PMS treatment: what actually helps.