Evening primrose oil side effects, including mood

Most people who take evening primrose oil notice only mild side effects, if any. Across clinical trials covering 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, and the review itself didn't report systematically on adverse events across the trials it covered. The review didn't set out to track side effects systematically, 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, and and didn't address safety as a specific outcome in the studies it covered.

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 the ones that measured bloating and fluid retention as a symptom category 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 rather than common. It's a mild blood thinner, so doctors usually advise stopping it before surgery, and the same caution applies if you're already on anticoagulant medication.

People with a seizure disorder are often told to avoid it, since gamma-linolenic acid, its main active compound, has a theoretical link to a lowered seizure threshold. Pregnancy is also a reason for caution, since 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. A 1988 trial in Human Psychopharmacology focused specifically on women with PMS-related anxiety and depression, the group most likely to notice a mood effect if there was one. Depression and anxiety scores improved over the two months of the trial, but they improved just as much on placebo as on evening primrose oil.

Larger reviews of evening primrose oil for PMS land in the same place. A 1996 systematic review found the two best-controlled trials showed no benefit for psychological symptoms, and a 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 on a hard day, 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, 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 (which some people take only in the second half of the cycle), certain hormonal contraceptives, and 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, our dosage guide covers how much and when, and our page on PMS supplements puts it alongside the other options the evidence actually supports. For a closer look at what it can and can't do, see our pages on evening primrose oil for PMS and evening primrose oil for PMDD. Many people try a supplement for a few cycles, track what actually changes, and adjust from there.