Evening primrose and borage oils have an unusually good tolerance profile for a supplement: in trials, their adverse effects are the same as those of their placebos, in both frequency and intensity. This is not a polite formula, it is what the Cochrane review reports in black and white after examining them across 27 trials.
The risk, when it exists, does not come from the oil taken in isolation. It comes from two specific situations and a well-identified mechanism: a measured effect on platelets, rather favorable in a healthy woman, but which adds to a blood-thinning medication. This article details what the trials actually observed, what Cochrane says about prolonged use, why pregnancy is excluded, and what the epilepsy warning found everywhere is worth.
The common risk. Low. The meta-analysis of 13 trials and 1,752 patients finds no increase in nausea, bloating, headaches, dizziness, weight gain, or taste disturbance compared to placebo. The real drawback is oily reflux, which taking it with meals almost always resolves.
The mechanism to know. A trial in postmenopausal women shows that evening primrose oil reduces platelet aggregation, dense granule secretion, and platelet integrin activation, with effects that persist after discontinuation. Cardioprotective in a healthy woman, additive with an anticoagulant. The two exclusions: Pregnancy and breastfeeding, anticoagulant or antiplatelet treatment without medical advice. The gray area: Continuous use beyond one year, not studied, with one published case that suggests preferring courses of treatment.
Real adverse effects, quantified
In 13 randomized trials totaling 1,752 patients, evening primrose oil does not increase adverse events such as nausea, abdominal bloating, headaches or dizziness, nor weight gain or taste alteration, compared to placebo or other comparison treatments. The authors qualify evening primrose oil as safe medication.
Ahmad Adni LL, Norhayati MN, Mohd Rosli RR, Muhammad J. Int J Environ Res Public Health 2021;18(12):6295. DOI: 10.3390/ijerph18126295
The Cochrane review dedicated to eczema, which examined 27 trials and 1,596 participants, formulates the same observation in another way: in these studies, evening primrose oil, borage oil and their placebos shared the same adverse effects, quite common, mild, transient and mainly digestive. In other words, part of what participants attributed to the oil resulted from the act of taking a capsule, not its content.
The clinical study involving 1,015 patients completes the list with rarer events: allergy, anxiety, blurred vision, constipation and nausea. They were infrequent and showed no statistical difference compared to the comparison group, but they exist and merit being known.
Oil aftertaste, that taste that returns one to two hours after taking it. It is nothing dangerous and is almost always resolved the same way: take the capsules in the middle of a meal rather than at the beginning or on an empty stomach, and with a glass of water. The fat in the meal slows gastric emptying and the oil does not rise back up. If the problem persists, taking one capsule at lunch and one at dinner works better than a single dose.
Platelets: the central mechanism
Postmenopausal women were supplemented with fish oil or evening primrose oil, with evaluation of platelet aggregation, dense granule secretion and alpha-IIb-beta-3 integrin activation, at rest and in response to an agonist. Evening primrose oil supplementation attenuated thrombin receptor PAR4-induced platelet aggregation, decreased dense granule secretion and attenuated basal integrin activation. After the withdrawal period following evening primrose oil supplementation, aggregation remained comparably attenuated, suggesting a sustained shift in platelet reactivity.
Yamaguchi A, Stanger L, Freedman JC, et al. Clin Transl Sci 2022;15(10):2378-2391. DOI: 10.1111/cts.13366
This trial is interesting in two respects. Its authors present it as a favorable argument: postmenopausal women have increased cardiovascular risk partly linked to platelet hyperactivity, and they conclude that supplementation with fish oil or evening primrose oil supports the idea of a therapeutic intervention to reduce the risk of thrombotic events in this population.
But the same mechanism explains the main precaution. If evening primrose oil reduces platelet aggregation, it does, on a small scale, what an antiplatelet agent does. The two add together. And the detail that matters for practice is that the effect persisted after discontinuation : it is not an action that disappears the day after the last capsule, which justifies discontinuing in advance before a surgical procedure rather than the day before.
Medications not to combine
| Class | Examples | Why |
|---|---|---|
| Anticoagulants (Vitamin K antagonists) | Warfarin, fluindione, acenocoumarol | The Cochrane review reports a study showing an increase in bleeding with warfarin |
| Direct oral anticoagulants | Apixaban, rivaroxaban, dabigatran | Addition with the measured effect on platelets |
| Antiplatelet agents | Aspirin at antiplatelet dose, clopidogrel, ticagrelor | Same target, effects that accumulate |
| Heparins | Treatment or prevention in the post-operative period | Context where hemorrhagic risk is already monitored |
| Other antiplatelet supplements | High-dose omega-3, garlic, ginkgo, concentrated turmeric | The combination of multiple supplements acting on platelets is rarely taken into account |
You don't need to remember this list. A single question to the pharmacist is enough before purchasing: "I'm taking this, can I take a supplement that slightly thins the blood?". If you don't take any regular medication, are neither pregnant nor breastfeeding and no surgery is planned, the question doesn't arise. The same reasoning applies moreover to ginkgo and to omega-3 at high dose.
Pregnancy, breastfeeding, late pregnancy
A reference review published in American Family Physician concludes that the use of evening primrose oil during pregnancy is not supported by the literature and should be avoided. There are insufficient safety data, and the documented effect on platelets constitutes an additional argument for caution as delivery approaches. Breastfeeding has not been studied any further.
Traditional use holds that evening primrose oil prepares the cervix in late pregnancy and facilitates labor onset. This practice circulates widely, including among expectant mothers. Trials do not support it.
Eighty nulliparous low-risk women at 40 weeks of pregnancy with a Bishop score below 4 received 1,000 mg of evening primrose oil twice daily for 7 days or an identical placebo, with follow-up until delivery. The two groups did not differ significantly in gestational age at delivery, need for labor induction or acceleration, duration of the different phases of labor, newborn weight, or Apgar scores. The authors conclude that there is insufficient evidence for the effectiveness of evening primrose oil for cervical maturation and labor duration, and suggest limiting its use to clinical trials and preventing its use in routine practice.
Kalati M, Kashanian M, Jahdi F, Naseri M, Haghani H, Sheikhansari N. J Obstet Gynaecol 2018;38(4):488-492. DOI: 10.1080/01443615.2017.1386165
A nuance deserves to be noted for accuracy: a 2023 trial compared, in 40 patients candidates for hysteroscopy or curettage, 2,000 mg of evening primrose oil by vaginal route to 200 micrograms of vaginal misoprostol two hours before the procedure, and found the first significantly more effective for cervical preparation, with less pain. This is a supervised gynecological context, a different route of administration, and a scheduled procedure, unrelated to oral intake of capsules during pregnancy.
Prolonged use, the real gray area
The Cochrane review is explicit on this point, and its wording is worth repeating: the short-term studies it included do not examine the possible adverse effects of prolonged use of evening primrose oil or borage oil. In this regard, it cites a published case warning that if taken for more than one year, there would be a potential risk of inflammation, thrombosis, and immunosuppression. It concludes its review by noting that the journal does not provide information on long-term use of these products.
This must be read for what it is: neither a health alert nor a blank check. A single case does not establish a risk, but the absence of data beyond six months does not establish safety either. The longest published trials last six months, most between six and twelve weeks.
Courses of two to three months followed by a window of a few weeks cover the duration over which benefits have been measured, including the twelve weeks of the skin trial, while remaining within the studied parameters. It is also a useful opportunity to verify whether the effect obtained is maintained upon stopping, which provides better information than continuous intake out of habit.
The warnings in circulation
Epilepsy
This is the most common precaution cited on product sheets and in monographs: evening primrose oil would lower the epileptogenic threshold. We searched for it. Our search on PubMed found no clinical trial or case series convincingly establishing this risk. We can therefore neither confirm nor rule it out, and we prefer to state this rather than copy a warning whose source we cannot find.
In practice, this does not change the recommended course of action: a person with epilepsy under treatment should in any case have any supplement validated by their neurologist, which settles the question without needing to resolve the literature.
Borage alkaloids
The borage plant naturally contains pyrrolizidine alkaloids in its aerial parts, leaves and flowers, compounds that should be avoided. This is why consumption of borage leaves in large quantities is not recommended.
Oil pressed from seedsis practically free of them or contains only trace amounts, and oils intended for dietary supplements are subject to controls on this specific point. The distinction between the plant and seed oil is therefore important, and this is another reason to choose a traceable oil whose manufacturer documents their analyses rather than the cheapest option on the shelf.
The checklist before starting
60 capsules, four lines of composition: organic evening primrose oil, organic borage oil, natural tocopherols, capsule shell. First cold pressing to preserve polyunsaturated fatty acids, natural vitamin E to protect them from oxidation. Two capsules during a meal, 30-day course.
View Evening Primrose / Borage OilNot recommended for pregnant or breastfeeding women and people under anticoagulant treatment. Do not exceed the recommended dose.
Choose the situation that applies to you: the answer will appear just below.
Nothing prevents it. Two capsules in the middle of a meal, in a course of two to three months. If oily aftertaste bothers you, take one capsule at lunch and one at dinner. The doses and expected benefits are detailed in the complete guide.
The antiplatelet effect of evening primrose oil is measured, not theoretical, and it adds to your treatment. The Cochrane review also reports a study showing an increase in bleeding under warfarin. Show this article to your doctor or pharmacist: the decision is theirs.
Lack of safety data, and a reference review concluding that use during pregnancy should be avoided. This includes the traditional use in late pregnancy to prepare the cervix: the randomized trial of 80 women at 40 weeks found no effect, and its authors recommend preventing this common practice.
One to two weeks before, and notify during the preoperative consultation. The important point is that in the platelet trial, the effect persisted even after the withdrawal period: stopping the day before is not enough. This also applies to tooth extraction.
This test provides guidance; it does not replace professional health advice.
Frequently Asked Questions
Tolerability
Is evening primrose oil dangerous?
In a person without treatment and outside of pregnancy, it is well tolerated. The meta-analysis covering 13 trials and 1,752 patients found no increase in nausea, bloating, headaches, dizziness, weight gain, or taste disturbance compared to placebo or comparison treatments. The risk does not come from the oil itself but from two situations: pregnancy and blood-thinning treatments.
What are the most common side effects?
Mild and transient digestive disturbances, essentially oily reflux and gastric discomfort, more rarely headaches. The Cochrane review highlights an interesting point: in trials, evening primrose oil, borage oil, and their placebos shared the same adverse effects, with the same frequency and intensity. Taking capsules during a meal significantly reduces reflux.
Is there a risk of allergy?
It is rare but possible, as with any plant. In the clinical study involving 1,015 patients, allergy was among reported effects, alongside anxiety, blurred vision, constipation, and nausea, but these events were rare and showed no statistical difference compared to the comparison group. A skin reaction, swelling, or breathing difficulty after intake requires stopping and seeking medical advice.
Interactions
Why are anticoagulants a contraindication?
Because the effect on platelets is real and measurable. A trial in postmenopausal women showed that evening primrose oil supplementation attenuates PAR4 receptor-induced platelet aggregation, reduces dense granule secretion, and decreases basal platelet integrin activation. These effects persisted even after the washout period. In a healthy woman, this is rather a cardioprotective signal; when combined with a medication that already thins the blood, these effects add up.
What medications require medical advice before taking it?
All those that act on coagulation or platelet aggregation: direct oral anticoagulants, antivitamin K drugs such as warfarin, aspirin at antiplatelet doses, clopidogrel and related drugs. The Cochrane review reports a study showing increased bleeding in people on warfarin. In case of doubt about a treatment, consult the pharmacist before purchase, not after.
Should you stop before surgery?
It is prudent, for the same reason as with anticoagulants. The usual rule for supplements acting on platelets is to suspend them one to two weeks before planned surgery, and to inform the anesthetist during the preoperative consultation. This also applies to tooth extraction or an invasive procedure.
Pregnancy
Can you take it while pregnant?
No. A reference review published in American Family Physician concludes that use during pregnancy is not supported by the literature and should be avoided. There is insufficient safety data, and the measured effect on platelets is an additional argument for caution as delivery approaches. Breastfeeding has not been studied either.
Does evening primrose oil help induce labor late in pregnancy?
Trials do not show this. A randomized, triple-blind trial in 80 women at 40 weeks of pregnancy, with 1,000 mg twice daily for 7 days, found no difference in Bishop score, delivery timing, need for labor induction or acceleration, duration of different phases, newborn weight, or Apgar scores. The authors recommend limiting its use to clinical trials and avoiding common practice.
Duration and Quality
Can you take it year-round without interruption?
Clinical trials typically last from six weeks to six months, which limits our knowledge. The Cochrane review is explicit on this point: the short-term studies included do not examine possible adverse effects from long-term use, and it mentions a published case suggesting, after more than one year of continuous use, a potential risk of inflammation, thrombosis, and immunosuppression. Courses of two to three months with breaks remain the best-documented framework.
Is there a particular risk with borage oil?
The borage plant naturally contains pyrrolizidine alkaloids in its aerial parts, compounds to be avoided. The oil pressed from the seeds is practically devoid of them or contains them only in trace amounts, and oils intended for supplements are subject to testing on this point. This is one more reason to favor a traceable oil, whose manufacturer documents their analyses, rather than the cheapest option.
What should one think of the warning about epilepsy?
This precaution circulates widely in monographs and product sheets, often without reference. Our search on PubMed did not find a clinical trial or case series convincingly establishing this risk. We therefore can neither confirm nor rule it out. In practice, a person with treated epilepsy should in any case validate any supplement with their neurologist, which settles the question without having to decide the literature.
What should cause immediate discontinuation?
Unusual bleeding, repeated nosebleeds, bruises that appear easily, bleeding gums, or an allergic reaction with rash, swelling, or breathing difficulty. These signs require stopping and seeking medical advice, mentioning everything you are taking, including supplements.
Glossary
- Platelet aggregation
- Clustering of blood platelets, the first stage of clot formation.
- Antiplatelet agent
- A substance that reduces platelet aggregation; low-dose aspirin is the common example.
- Anticoagulant
- A medication that slows blood coagulation through a mechanism other than action on platelets.
- Platelet integrin
- Surface receptor of platelets whose activation allows their mutual adhesion.
- Bishop score
- Obstetric score assessing cervical maturity before labor induction.
- Pyrrolizidine alkaloids
- Compounds present in the aerial parts of several plants including borage, potentially toxic to the liver; absent or present in trace amounts in seed oil.
- Tocopherol
- Natural form of vitamin E, used to protect polyunsaturated oils from oxidation.
- Cochrane Review
- Methodologically rigorous synthesis of all available trials on a question, reference standard in evidence-based medicine.
Sources
The studies cited below were identified via PubMed.
- Bamford JTM, Ray S, Musekiwa A, van Gool C, Humphreys R, Ernst E. Evening primrose oil and borage oil orally in eczema. Cochrane Database of Systematic Reviews, 2013;(4):CD004416. DOI
- Ahmad Adni LL, Norhayati MN, Mohd Rosli RR, Muhammad J. Systematic review and meta-analysis of the efficacy of evening primrose oil in the treatment of mastalgia. International Journal of Environmental Research and Public Health, 2021;18(12):6295. DOI
- Yamaguchi A, Stanger L, Freedman JC, et al. Omega-3 or omega-6 fatty acid supplementation attenuates platelet reactivity in menopausal women. Clinical and Translational Science, 2022;15(10):2378-2391. DOI
- Bayles B, Usatine R. Evening primrose oil. American Family Physician, 2009;80(12):1405-1408. PubMed
- Kalati M, Kashanian M, Jahdi F, Naseri M, Haghani H, Sheikhansari N. Evening primrose oil and labor, is it effective? Randomized clinical trial. Journal of Obstetrics and Gynaecology, 2018;38(4):488-492. DOI
- Rahbar N, Sharafshahi F, Ghods S, Ghorbani R. Comparison of misoprostol and evening primrose oil for cervical preparation before gynecological surgery. Journal of Clinical Pharmacology, 2023;63(8):880-885. DOI
- Balci FL, Uras C, Feldman S. Clinical factors affecting the therapeutic efficacy of evening primrose oil on mastalgia. Annals of Surgical Oncology, 2020;27(12):4844-4852. DOI
- Sharifi M, Nourani N, Sanaie S, Hamedeyazdan S. Effect of evening primrose oil on inflammatory diseases: a systematic review of clinical trials. BMC Complementary Medicine and Therapies, 2024;24(1):89. DOI
