Omega-3 and evening primrose oil for PMS: what the trials found

Omega-3s are fats found mainly in fish oil. The main types used in supplements are EPA and DHA, and a 2018 trial used about 1 gram of fish oil a day; the trial didn't report how much of that was EPA and DHA specifically (Journal of Psychosomatic Obstetrics and Gynaecology, 2018). A typical over-the-counter fish oil softgel contains a similar amount, though labels vary by brand.

Evening primrose oil (EPO) comes from the seeds of a plant called Oenothera biennis. It contains linoleic acid and gamma-linolenic acid (GLA), an omega-6 fat once thought to ease PMS by correcting a fatty-acid imbalance.

Nordic Naturals Omega Woman combines fish oil with evening primrose oil in one capsule. No trial we found tested that exact combination, so the evidence below covers the two ingredients on their own, not the packaged product.

Omega-3 and evening primrose oil have been tested separately for PMS, and the results point in different directions.

A 2022 systematic review and meta-analysis of three randomized trials found omega-3 reduced PMS severity, including both physical and mood symptoms (Journal of Obstetrics and Gynaecology Research, 2022). The same review flagged high variation between the trials, so the size of the effect is uncertain.

Evening primrose oil has a longer research history and a weaker result. A 1996 systematic review of seven placebo-controlled trials found the two best-designed studies showed no benefit over placebo (Controlled Clinical Trials, 1996). A later six-cycle crossover trial reached the same conclusion (The Medical Journal of Australia, 1990).

On the evidence available, omega-3 has the stronger case, though neither is a proven PMS treatment on its own.

The trials behind these numbers studied PMS, not PMDD. None of the omega-3 or evening primrose oil trials we found required a confirmed PMDD diagnosis using daily symptom ratings.

That distinction matters. PMDD is a DSM-5-TR diagnosis that needs at least five symptom categories, including a mood symptom, confirmed by two cycles of daily tracking. PMS is broader and doesn't need that level of confirmation.

So what we can say is narrower than it sounds: omega-3 might ease general PMS symptoms, but whether it helps the mood symptoms that define PMDD specifically hasn't been tested in the research we reviewed. If you're trying to work out which pattern fits you, the DRSP chart uses the same kind of daily tracking researchers rely on.

For a closer look at each condition on its own, see omega-3 for PMS and omega-3 for PMDD.

There's no official dose for omega-3 or evening primrose oil in PMS guidelines. What we have is what researchers used in trials.

• Omega-3 (fish oil): about 1 gram of fish oil a day in the main PMS trial, though the trial didn't specify its EPA/DHA content (Journal of Psychosomatic Obstetrics and Gynaecology, 2018)
• Evening primrose oil trials used varying doses, commonly a few grams a day, continued for several months, since one review found it needs 4 to 6 months of regular use to judge a response (Journal of Menopausal Medicine, 2019)
• Combined products like Omega Woman list their own amounts on the label. No trial tested that specific combination, so there's no research-based dose for the product as a whole.

Both supplements are usually well tolerated, and the side effects reported in trials were mild rather than serious.

Omega-3 (fish oil) can cause:

• A fishy aftertaste or burping after taking it
• Mild stomach upset or nausea, especially on an empty stomach
• Loose stools if the dose is high

Evening primrose oil can cause:

• Headache
• Mild stomach upset
• Occasional nausea

None of the trials we reviewed reported serious adverse effects from either supplement at normal doses. Most side effects showed up early and eased within a week or two as the body adjusted. If stomach upset or headache doesn't settle after that, it's reasonable to stop taking it.

Both can interact with certain medications, mostly ones that affect bleeding.

Omega-3 at higher doses can thin the blood slightly, which matters if you're taking warfarin, aspirin or other blood thinners. Evening primrose oil has a similar mild effect and is sometimes advised against before surgery.

Neither supplement has been tested for interactions with hormonal contraception or SSRIs, the treatments usually recommended for PMDD.

If your symptoms are severe enough to disrupt a week of every month, see a GP or gynecologist and bring two months of daily notes. If you want someone who specializes in this, finding a PMDD specialist is a good place to start.

Put together, the picture is mixed rather than settled.

Omega-3 has one meta-analysis and one sizable trial pointing toward a real, if uncertain, benefit for PMS (Journal of Obstetrics and Gynaecology Research, 2022; Journal of Psychosomatic Obstetrics and Gynaecology, 2018).

Evening primrose oil has more trials behind it, and most of the better-designed ones found no benefit beyond placebo (Controlled Clinical Trials, 1996; The Medical Journal of Australia, 1990; Human Psychopharmacology, 1988). A 2024 systematic review of EPO across several conditions reached the same conclusion for PMS specifically.

Neither ingredient replaces treatments with stronger guideline support, like certain SSRIs, some hormonal contraceptives or CBT. For a closer look at what helps, see the PMS treatment guide and the PMS and PMDD research library.