Evening primrose oil is well tolerated by most people, and it has 2 cautions that genuinely matter, and 1 more in pregnancy. Both involve medication rather than the oil itself.
The first is bleeding risk with anticoagulants. The second is a historical seizure concern, and pregnancy makes a third category of its own.
Quick Answer: What Are the Side Effects of Evening Primrose Oil?
Mild digestive upset, nausea and headache are the common ones, and they are usually dose-related. The two cautions that matter involve medication: a bleeding-risk concern with anticoagulants and antiplatelet drugs, and a historical seizure caution, particularly alongside phenothiazines. Pregnancy is a separate conversation with a midwife or obstetrician, and only 176 women have been studied there.
Key Takeaways
- Digestive upset and headache are common, and both track the 1 dose taken.
- There are 2 medication cautions: anticoagulants and the seizure concern.
- Phenothiazine medicines are named in that 1 old seizure caution too.
- Pregnancy is a maternity conversation rather than 1 for a label.
- Stop before any surgery, given that same 1 bleeding-risk concern.
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The Everyday Effects
For most people taking a softgel or two a day, the reality is unremarkable.
- Stomach upset: The most common, and it eases with food.
- Nausea: Usually dose-related.
- Headache: Reported, and not consistently.
- Loose stools: More likely at the higher trial doses.
The breast-pain trial ran 3000 mg daily for 6 months in an academic setting, which is a fair test of tolerance. What the oil is comes first, in our guide to the oil itself.
The Two Medication Cautions
These are the parts of the label worth reading twice.
| Caution | Who it applies to | What to do |
|---|---|---|
| Bleeding risk | Warfarin, other anticoagulants, antiplatelets | Ask before starting |
| Bleeding risk | Anyone due for surgery | Mention it and stop in advance |
| Seizure concern | Anyone with a seizure disorder | Avoid unless cleared |
| Seizure concern | Anyone taking phenothiazines | Avoid unless cleared |
Neither caution comes from a large modern trial, and both are longstanding enough that a pharmacist will recognise them. Ask rather than guess.
Pregnancy
This is the most consequential section of the hub, and the position is simple.
- Who decides: Your midwife or obstetrician.
- Oral evidence: 1 trial, and it found no effect at all.
- Vaginal evidence: A 0.64-point Bishop score difference.
- Total women studied: 176 across both trials.
The oral trial was triple-blind in 80 nulliparous women and found no difference in gestational age at delivery, induction need, labour duration, birth weight or Apgar scores.[1]Evening primrose oil and labour, is it effective? A randomised clinical trial — Journal of Obstetrics and Gynaecology (2018) View source Both trials are set out in the labour trials, by route.
How Safe Is It Otherwise
The size of the evidence base cuts both ways: it limits confidence about benefits and about harms equally.
| Evidence | Scale |
|---|---|
| Cochrane eczema review | 27 studies, 1596 participants |
| Breast pain trial | 85 enrolled, 41 completed, 6 months |
| Labour trials | 176 women across 2 trials |
| 2024 systematic review | Highly heterogeneous, no strong recommendations |
| Long-term studies | None running for years |
Cochrane conducted a separate search specifically for adverse effects of evening primrose oil and borage oil, alongside the efficacy analysis.[2]Oral evening primrose oil and borage oil for eczema — Cochrane Database of Systematic Reviews (2013) View source That is a reassuring process even when the efficacy result is negative.
Who Should Simply Skip It
A short list, and the first 2 entries cover most of the risk.
- On anticoagulants: Unless a clinician has cleared it.
- Seizure disorder: Or taking phenothiazines.
- Pregnant: Unless your maternity team has decided otherwise.
- Due for surgery: Stop in advance and tell the team.
Amounts and how to start are in the dosage guide, and the honest ceiling on what to expect is low.
A 2024 systematic review concluded the literature on this oil is highly heterogeneous and fails to support strong recommendations in either direction.[3]The effect of Oenothera biennis (Evening primrose) oil on inflammatory diseases: a systematic review of clinical trials — BMC Complementary Medicine and Therapies (2024) View source
- Common effects: Mild, and usually eased by food.
- Real cautions: The 2 medication ones above.
- Pregnancy: A maternity conversation, every time.
For everyone else, the softgel we sell taken with food has an undramatic record.
Contact a Clinician or Maternity Unit Rather Than Continuing If
- Any plan to use evening primrose oil to start labour or ripen the cervix without your midwife or obstetrician deciding it, including vaginal use
- Reduced fetal movements, bleeding, fluid loss or regular painful contractions in pregnancy
- Taking warfarin or any anticoagulant or antiplatelet medicine
- A seizure disorder, or taking phenothiazine medicines
- A new breast lump, skin change or nipple discharge
Frequently Asked Questions
What are the bad side effects of evening primrose oil? +
Stomach upset, nausea and headache, and they are usually dose-related. The 2 cautions that matter more involve anticoagulants and seizures.
What to avoid while taking evening primrose oil? +
Anticoagulant and antiplatelet medicines without asking first, and phenothiazines if you have a seizure disorder. Those are the 2 named concerns.
Who should avoid evening primrose oil? +
Anyone on anticoagulants, anyone with a seizure disorder, anyone pregnant who has not asked, and anyone due for surgery. That is 4 groups.
Who should not use evening primrose? +
The same 4 groups. For everyone else the effects reported are mild and usually settle with food.
Does it thin the blood? +
The bleeding-risk concern is why it is flagged with anticoagulants. Ask a pharmacist before combining the 2.
Is it safe to take every day? +
The breast-pain trial ran 3000 mg daily for 6 months in an academic setting. There are 0 studies running for years.
What happens when you take evening primrose oil every day? +
For most people, very little. The largest evidence base, 27 eczema studies in 1596 people, found no significant change at all.
Is it safe in pregnancy? +
The 2 pregnancy trials together covered 176 low-risk women, which is too small a base for a general statement. Ask your maternity team.
Can it interact with the contraceptive pill? +
There is no established interaction. The 2 documented cautions concern anticoagulants and seizure risk instead.
Should I stop before an operation? +
Yes, mention it and stop in advance. That is standard for oils carrying a bleeding-risk concern, and it applies to all 3 of them.
Does it cause weight gain? +
There are 0 trials showing that. Weight was not an outcome in any of the studies covered here.
Can teenagers take it? +
Some of the eczema trials included children among their 1596 participants. It is still a clinician question rather than a shelf decision.
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