Evening Primrose (Oenothera biennis): Benefits, Dosage, Safety & Evidence

What is a herb–drug interaction?

A herb–drug interaction happens when a plant or supplement changes how a medication works in your body — making it stronger, weaker, or more likely to cause harm. Herbs contain active compounds that can speed up or slow down the enzymes and transporters your body uses to process drugs.

Why it matters

St. John's Wort can make birth control, antidepressants, and blood thinners less effective.

Garlic can increase bleeding risk with warfarin and other blood thinners (also ginkgo, ginger, and turmeric).

Kava can amplify sedation when taken with anxiety or sleep medications (also valerian).

The risk is highest for older adults, anyone taking several medications, and people with chronic conditions.

Always tell your doctor and pharmacist about every herb and supplement you take — and check below before combining.

Known Drug Interactions (2)

1 Severe1 Moderate

Drug	Severity	Description
Warfarin (Coumadin)	Moderate	Evening primrose oil may have mild anticoagulant properties that add to warfarin's effect.
Phenothiazines (Chlorpromazine)	Severe	Evening primrose oil may lower the seizure threshold, increasing seizure risk in patients taking phenothiazines.

This list reflects a curated subset of documented interactions and is not exhaustive. The absence of a drug here is not proof that it is safe to combine. Always confirm with a pharmacist or healthcare provider.

Frequently asked questions

How does evening primrose oil work in the body?

Evening primrose oil works primarily through its high gamma-linolenic acid (GLA) content, an omega-6 fatty acid. GLA is converted in the body to prostaglandin E1, which has anti-inflammatory and hormone-regulating properties. This conversion occurs via the delta-6 desaturase enzyme pathway, which can be impaired in certain health conditions. The resulting prostaglandins help modulate inflammation, support skin barrier function, and regulate hormonal responses. Evidence level: B for mechanism understanding.

Does evening primrose oil interact with any medications?

Yes, evening primrose oil has clinically significant drug interactions. It may increase bleeding risk when combined with anticoagulants (warfarin, heparin), antiplatelet agents (aspirin, clopidogrel), or NSAIDs. It should be avoided with phenothiazine medications used for schizophrenia, as this combination may increase seizure risk. Patients on anticonvulsant medications should use caution. Due to potential bleeding effects, evening primrose oil should be discontinued at least two weeks before scheduled surgery. Evidence level: C for interactions.

What is the recommended dosage for evening primrose oil?

The standard adult dosage of evening primrose oil is 2-8 grams daily, taken in divided doses with meals to enhance absorption. This provides approximately 240-960mg of gamma-linolenic acid (GLA) per day. For cyclic breast pain (mastalgia), clinical studies have typically used 3-4 grams daily. For premenstrual symptoms, 2-4 grams daily is common. Therapeutic effects may require 3-4 months of consistent use. Start with lower doses to assess individual tolerance. Evidence level: B for dosing recommendations.

Is evening primrose oil safe to take during pregnancy?

Evening primrose oil is considered unsafe during pregnancy and is not recommended. Although some traditional use exists for cervical ripening near term, there is insufficient evidence for safety. The oil may stimulate uterine contractions or increase bleeding risk during delivery. Pregnant women should avoid evening primrose oil throughout all trimesters. Women who are breastfeeding should consult a qualified healthcare provider before use, as safety data during lactation is limited. Evidence level: D for pregnancy safety.

Can evening primrose oil help with breast pain and PMS symptoms?

Evening primrose oil has been extensively studied for cyclic breast pain (mastalgia) and premenstrual symptoms. The GLA content supports prostaglandin synthesis, which may reduce breast tenderness and swelling. Evidence is stronger for breast pain, with Commission E approval for this indication, while PMS results remain mixed across clinical trials. Benefits typically require 3-4 months of consistent daily use. Individual response varies considerably, and not all users experience symptom improvement. Evidence level: B for breast pain, C for PMS.

PubMed-compiled information sheet

This sheet was compiled from PubMed (NIH) abstracts using AI assistance. Every factual claim is cited to a real PubMed article (see the source list). It has not yet been human-reviewed — confirm with a healthcare provider before use.

Compiled from 29 PubMed articles · model: gemma4:31b

Summary

Evening primrose (Oenothera biennis) is a plant source of omega-6 essential fatty acids used primarily for inflammatory conditions and women's health [PMID:20000302, PMID:37614101].

Background

Evening primrose (Oenothera biennis) is a plant in the Onagraceae family [PMID:30110920]. It is commonly used as an alternative therapy and is a rich source of omega-6 essential fatty acids [PMID:20000302].

Traditional uses

It is believed in traditional medicine to facilitate labor [PMID:24535321].

Active compounds

The seeds contain a fixed oil (EPO) rich in linoleic acid (60%-80% or 70-74%) and gamma-linolenic acid (GLA) (8%-14% or 8-10%) [PMID:30110920, PMID:31497576, PMID:37614101]. Other components include aliphatic alcohols, sterols, polyphenols, triterpenes, phenolic acids, tocopherols, and phytosterols [PMID:30110920, PMID:37614101].

Clinical evidence

Evidence AMastalgia (Breast Pain)

Mixed results: some sources suggest symptoms may be attenuated [PMID:12391713], while a meta-analysis of 13 trials found no difference compared to placebo [PMID:34200727] and other reviews suggest it is likely ineffective [PMID:20000302].

Evidence AMenopausal Hot Flashes

A systematic review found that severity of hot flashes was lower when EPO was administered for less than 6 months compared to placebo, though no significant difference was found in frequency and duration [PMID:39829189].

Evidence AAtopic Dermatitis/Eczema

Current evidence suggests oral EPO does not provide clinically significant improvement [PMID:20000302].

Evidence CPremenstrual Syndrome (PMS)

Likely ineffective for the treatment of premenstrual syndrome [PMID:20000302].

Evidence BCervical Ripening/Parturition

Vaginal EPO (2000 mg) showed significantly greater cervical dilation (larger Hegar dilator size) compared to vaginal misoprostol (200 μg) before gynecological surgery [PMID:37102338]. In missed abortion, addition of 2000 mg vaginal EPO to misoprostol showed significant differences in full abortion and cervical dilatation [PMID:38089421].

Evidence A

Safety & adverse effects

Evening primrose oil is 'generally recognized as safe' (GRAS) as a food by the U.S. FDA [PMID:30000954]. Reported adverse events in peripartum persons were generally mild [PMID:36548328].

Pregnancy & lactation

Some suggest intake throughout pregnancy to prevent preterm delivery and promote easier birth [PMID:15124319], but other reviews state very few scientific data support the facilitation of labor and note associations with severe adverse events [PMID:24535321]. In nursing mothers, EPO increases breastmilk content of linoleic acid and total GLA without causing adverse reactions in breastfed infants [PMID:30000954].

Evidence summary

The evidence is mixed and often contradictory. While some systematic reviews and meta-analyses suggest ineffectiveness for mastalgia and atopic dermatitis, specific clinical trials show efficacy for cervical ripening. Evidence for menopausal symptoms and rheumatoid arthritis remains inconsistent.

PubMed sources

1.PMID: 34946512 (2021) — Herbal Products Used in Menopause and for Gynecological Disorders. · Molecules (Basel, Switzerland)
2.PMID: 30000954 (2006) — A review of effective herbal medicines in controlling menopausal symptoms. · Electronic physician
3.PMID: 20000302 (2009) — Evening primrose oil. · American family physician
4.PMID: 34957578 (2022) — An updated review on efficacy and benefits of sweet almond, evening primrose and jojoba oils in skin care applications. · International journal of cosmetic science
5.PMID: 21477394 (2011) — Breast pain. · BMJ clinical evidence
6.

Evening Primrose

Sources & Citations

Calculate a safe dose of Evening Primrose

Vitex Negundo

Red Clover

Black Cohosh

⚠️ Not Recommended During Pregnancy or Nursing

2 Known Drug Interactions — Some May Be Serious

Contraindications

Possible Side Effects

What is a herb–drug interaction?

Known Drug Interactions (2)

Frequently asked questions

Summary

Background

Traditional uses

Active compounds

Clinical evidence

Safety & adverse effects

Pregnancy & lactation

Evidence summary

PubMed sources

Government sources

Evening Primrose

Sources & Citations

Calculate a safe dose of Evening Primrose

Related Herbs

Vitex Negundo

Red Clover

Black Cohosh

⚠️ Not Recommended During Pregnancy or Nursing

2 Known Drug Interactions — Some May Be Serious

Contraindications

Possible Side Effects

What is a herb–drug interaction?

Known Drug Interactions (2)

Frequently asked questions

Summary

Background

Traditional uses

Active compounds

Clinical evidence

Safety & adverse effects

Pregnancy & lactation

Evidence summary

PubMed sources

Government sources