In studies, patients taking evening primrose oil have reported only minor side effects like headaches, nausea, upset stomach and loose stool [source: MedicineNet].
Because evening primrose oil may cause uterine contractions, it is not recommended for pregnant women. Patients with blood disorders or those taking blood thinners are also discouraged from using it [source: Memorial Sloan-Kettering Cancer Center].
In one study, researchers found it increased the risk of epilepsy in schizophrenic patients taking anti-psychotic medications like Thorazine and Prolixin. Another study found it could increase the number of seizures in patients with seizure disorder who underwent surgery requiring anesthesia [source: Mayo Clinic].
Remember, like all herbal remedies, evening primrose oil has not been approved by the FDA, which considers herbs dietary supplements rather than drugs. That's why they don't undergo rigorous clinical review before being released on the market. So like all things you put into your body, you should exercise caution. But if you're trying to get pregnant, you may want to throw caution to the wind -- at least where evening primrose oil is concerned. Read on to find out why.