Evening primrose (Oenothera biennis) is a traditional medicinal herb native to North America that now grows in many places worldwide. The plant is suitably named for its yellow flowers that bloom in the evening after sunset.^[3] Evening primrose oil (EPO) is extracted from the seeds of the evening primrose plant and is a rich source of omega-6 fatty acids, particularly linoleic acid (70–74%) and gamma-linolenic acid (GLA) (8–10%).^[2] While the fatty acid profile of EPO is generally thought to be the main factor driving its effects in the body, EPO also contains other bioactive compounds, including triterpenes, phenolic acids, and tocopherols (vitamin E).^[2] Despite being promoted as a remedy for a variety of ailments, most clinical research has not identified any clear benefits of EPO supplementation.

EPO has been used to improve cervical ripening/softness in preparation for childbirth. Current research does suggest that EPO (particularly when given vaginally) may improve Bishop scores — the gold standard assessment tool for predicting the success of labor induction — and may have comparable effects to the medication misoprostol.^[4][5][6][7] Some research has reported reductions in labor time and the rate of cesarean section with EPO use, but these findings are less consistent.^[6] Currently, there is no established dosing and large clinical trials are needed to properly assess the safety and efficacy of EPO in this context. Potential side effects that have been reported will be discussed in the following section.

EPO is generally safe and well-tolerated. The most common side effects include headache and gastrointestinal symptoms (e.g., heart burn, nausea, vomiting, diarrhea, abdominal pain, flatulence).^[8][9] When used prior to childbirth, reported side effects include meconium-stained amniotic fluid, premature membrane rupture, and arrest of descent;^[5] and in one case report, a woman using both EPO and raspberry leaf tea before labor gave birth to an infant with widespread bruising.^[10]

EPO might increase the risk of bleeding, particularly if combined with other blood-thinning medications (e.g., warfarin).^[11] Although evidence is limited, EPO may have an inhibitory effect on certain cytochrome P450 (CYP450) enzymes involved in drug metabolism, which could theoretically alter the efficacy or increase the risk of side effects with some medications; however, the clinical relevance of this is unclear.^[12] In one case report, EPO administration seemed to increase blood levels of an anti-retroviral medication (lopinavir/ritonavir) used for managing HIV, and the proposed mechanism was CYP450 enzyme inhibition.^[13]

Case reports have suggested that EPO may increase the risk of seizures.^[14][15] In all cases, EPO was administered to people with schizophrenia along with phenothiazine drugs, which are known to increase the risk of seizures, and all of the individuals had a history of seizures or abnormal brain wave activity. However, the mechanism behind this is unclear, and animal research suggests that the primary fatty acids in EPO (linoleic acid and GLA) and their metabolites may instead be protective against seizures.^[16][11]

The clinical effects of EPO are thought to be largely due to its content of the fatty acid GLA which can have immune-modulating and anti-inflammatory effects. While less thoroughly studied, EPO is also a source of polyphenols which likely contribute an anti-inflammatory and antioxidant benefit.^[2] In the body, GLA is a precursor for dihomo-gamma-linolenic acid (DGLA), which can in turn be metabolized into eicosanoids with anti-inflammatory properties, such as prostaglandin E1.^[17] Cervical ripening and labor induction during childbirth both depend on prostaglandins, and increased synthesis of prostaglandin E1 is thought to be the main mechanism by which EPO may increase cervical readiness.^[1] Similarly, misoprostol, a medication that is used to prepare the cervix for labor induction, is a prostaglandin E1 analog.^[18]