Evening primrose oil side effects, including mood

Most people who take evening primrose oil notice only mild side effects, if any. Across clinical trials for premenstrual syndrome, breast pain, eczema and other conditions, the complaints that come up most often are:

• An upset stomach
• Nausea
• Loose stools
• Occasional headache

A 2024 systematic review in BMC Complementary Medicine and Therapies looked at evening primrose oil across several inflammatory conditions and found mixed evidence for benefit. The review didn't report systematically on adverse events across the trials it covered, so this isn't a formal safety verdict.

A separate review of evening primrose oil in women's health, covering PMS, mastalgia and menopausal symptoms, found it generally needs four to six months of regular use before any benefit shows. It didn't address safety as a specific outcome.

That's the headline on safety. It doesn't mean the oil works for what you're taking it for, which is a separate question covered on our evening primrose oil overview page.

There's no trial evidence that evening primrose oil causes weight gain. None of the placebo-controlled PMS trials report it as a side effect, and trials measuring bloating and fluid retention found no real difference between evening primrose oil and placebo.

A 1990 crossover trial in the Medical Journal of Australia tracked ten PMS symptoms, including fluid retention, in 38 women over six menstrual cycles. Scores improved on both the active treatment and the placebo, with no significant difference between them.

That pattern, improvement that tracks with placebo rather than the active ingredient, shows up across most evening primrose oil trials for PMS.

If you're thinking of the bloated, heavier feeling that shows up before a period, that's a separate issue, a premenstrual symptom, not a side effect of a supplement. Our page on PMS treatment covers what actually helps with that.

For most healthy adults, evening primrose oil is considered low risk when taken at typical doses for a few months. The safety concerns that come up are specific, not common.

Situation	What to know
Having surgery	It's a mild blood thinner, so doctors usually advise stopping it first
Already on anticoagulant medication	The same caution applies
Seizure disorder	Often advised to avoid; gamma-linolenic acid, its main active compound, has a theoretical link to a lowered seizure threshold
Pregnant	Safety data is limited. One small trial gave evening primrose oil to women in the final week of pregnancy and found no difference in labor length or newborn outcomes compared to placebo, but that single short trial doesn't establish it as safe for routine use earlier in pregnancy

If you take prescription medication for a chronic condition, mention evening primrose oil to whoever prescribes it. It's about taking a supplement the same way you'd take anything else that affects your body, with a clear sense of what it does and who should check first.

Evening primrose oil hasn't been shown to reliably change mood in either direction.

Year	Study	Finding
1988	Trial in Human Psychopharmacology, in women with PMS-related anxiety and depression	Depression and anxiety scores improved over two months, but just as much on placebo as on evening primrose oil
1996	Systematic review	The two best-controlled trials showed no benefit for psychological symptoms
2022	Literature review	Described the results as contradictory, with no consistent pattern of improvement

So evening primrose oil hasn't been shown to worsen mood, and it hasn't been shown to reliably improve it either, beyond what a placebo does.

If what you're after is a way to manage the anger or sadness that shows up before your period, that's a different problem than finding the right supplement.

Niyora is a PMS and PMDD support app. Its Moon feature lets you write down what happened, see it from another angle, calm down, and draft a reply you can hold onto for 20 minutes.

When side effects happen, they tend to show up early and stay mild. Expect an upset stomach or headache in the first week or two, easing as your body adjusts or once you stop.

There's no trial data suggesting side effects build up or get worse the longer you take it.

Judging whether evening primrose oil is working is a separate timeline. A review of its use in women's health conditions found that an immediate response shouldn't be expected, and recommended regular use for four to six months before deciding whether it's helping.

If you've reached that point with no real change in your symptoms, it's reasonable to stop.

Evening primrose oil isn't the only option, and for premenstrual mood symptoms specifically, it isn't the one with the strongest evidence. Clinical guidelines for PMS and PMDD point to a short list of treatments with better trial support:

• SSRIs (some people take these only in the second half of the cycle)
• Certain hormonal contraceptives
• Cognitive behavioral therapy

A doctor decides what fits your situation and history.

One pocket of evidence looks a little better. A six-month, placebo-controlled pilot trial of evening primrose oil for cyclical breast pain found an improvement in worst-pain scores within the EPO group over time, but the difference from placebo wasn't statistically significant on direct comparison.

If breast tenderness before your period is your main complaint rather than mood or anger, that's worth knowing.

If you want to try evening primrose oil anyway, here's where to go next:

• Dosage guide: how much and when
• PMS supplements: how it compares to other options with stronger evidence
• Evening primrose oil for PMS
• Evening primrose oil for PMDD

Many people try a supplement for a few cycles, track what actually changes, and adjust from there.