Evening primrose oil vs fish oil for PMS

Evening primrose oil comes from the seeds of a yellow wildflower, Oenothera biennis. It's mostly linoleic acid and gamma-linolenic acid (GLA), both omega-6 fatty acids [4].

Fish oil is the common source of omega-3s: EPA and DHA, pulled from oily fish like salmon, mackerel or sardines. The two oils aren't variations on the same ingredient. They come from different fatty acid families entirely.

That matters because omega-6 and omega-3 fats are processed through some of the same pathways in the body, which is part of why researchers test them separately rather than as one combined remedy.

A quick side-by-side:

• Source: evening primrose oil comes from wildflower seeds; fish oil comes from oily fish such as salmon or mackerel.
• Fatty acid family: evening primrose oil is omega-6 (linoleic acid and GLA); fish oil is omega-3 (EPA and DHA) [4].
• What switching gives you: taking more evening primrose oil won't give you any omega-3s, and taking more fish oil won't give you any GLA.

If you want the full picture on evening primrose oil on its own, what evening primrose oil is and whether it helps PMS covers where it comes from and how it's typically used.

Evening primrose oil is an omega-6 oil. It is not an omega-3 source, despite often being shelved next to fish oil in the supplement aisle.

Its fatty acid makeup, from the clinical literature, breaks down roughly like this [4]:

• Linoleic acid: about 60% to 80% of the oil
• Gamma-linolenic acid (GLA): about 8% to 14%
• Omega-3 fatty acids: not present in a meaningful amount

GLA is the omega-6 fatty acid most of evening primrose oil's research has focused on [4]. Fish oil's EPA and DHA work through different anti-inflammatory routes.

Neither fatty acid family is better in general. They're different tools, tested for different things, and the research record for PMS specifically leans almost entirely on the omega-6 side, since that's what evening primrose oil trials have studied.

Most of the controlled research here is on evening primrose oil, and it isn't encouraging. The two best-designed trials, both randomized, double-blind and placebo-controlled, found no real difference between evening primrose oil and placebo for PMS symptoms [3] [5].

A systematic review of seven placebo-controlled trials concluded evening primrose oil is "of little value" for PMS, though the trials were small enough that a modest effect couldn't be fully ruled out [2]. A broader review of evening primrose oil across many inflammatory conditions reached the same conclusion for PMS specifically [1].

Larger trials out of Hamadan University (120 and 80 participants) also used double-blind, placebo-controlled designs but reported much bigger drops in symptoms with evening primrose oil [7] [8]. Why these results diverge so sharply from the smaller, placebo-controlled crossover trials [3] [5] isn't explained in the sources.

None of the trials reviewed here tested fish oil against placebo for PMS or PMDD, so there's no comparable evidence trail to weigh against it. For a closer look at the evidence for PMS and PMDD specifically, see evening primrose oil for PMS.

Evening primrose oil is generally well tolerated. Across the trials reviewed, the side effects reported were mild [1]:

• Nausea
• Headache
• Upset stomach or loose stools
• Occasional mild skin reactions

Fish oil's side effects aren't covered in the sources used for this page. Common reports describe a fishy aftertaste, burping or loose stools, usually eased by taking it with food.

Neither oil has a side effect profile that should stop you from asking a doctor about either one. The bigger open question is effectiveness, not safety.

For the fuller list of what evening primrose oil can cause, including effects on mood, see evening primrose oil side effects.

No trial in this review tested evening primrose oil and fish oil together for PMS, so switching or combining them isn't something the evidence can guide directly.

In practice, some people do take both, on the idea that balance between omega-6 and omega-3 matters more than either fatty acid alone. That idea is plausible, but it hasn't been tested for premenstrual symptoms in the trials cited here.

If you're currently on evening primrose oil and it isn't helping after a few cycles, switching to fish oil isn't backed by PMS-specific evidence either. Neither oil has a track record strong enough to promise a benefit if you switch.

Evening primrose oil's effects, when there are any, tend to show up slowly. One review found it should be used for four to six months before judging whether it helps [4]. For timing and dosing, evening primrose oil dosage and how long evening primrose oil takes to work go through what the trials used.

Given the evidence, evening primrose oil is reasonable to mention to a doctor, but it shouldn't replace treatments with stronger backing, like certain SSRIs or hormonal contraceptives, which guidelines support for PMS and PMDD. A doctor can help you weigh it alongside those options.

If your premenstrual symptoms are mild and mostly physical, like breast tenderness, evening primrose oil has slightly more support there than for mood symptoms [6]. If anger, sadness or anxiety are the main problem, the evidence for either oil is thin, and other approaches have more research behind them. See PMS treatment that actually helps for the fuller list.

While you're figuring out what helps, Niyora is a PMS and PMDD support app. Its Moon feature can help you write down what happened in a hard moment, see it from another angle, and calm down before you decide what to say or do next.