Evening primrose oil has a high concentration of gamma-linolenic acid (GLA) -- a fatty acid with a long name and an even longer list of functions it helps control in your body. When you eat food or take a supplement that contains this fatty acid, your body converts it into a hormone-like substance called prostaglandin E1, which stimulates contraction of the blood vessels [source: Whole Health MD].
Because of this anti-inflammatory effect, evening primrose oil has been used to treat a wide range of aches and pains caused by swelling, such as bloating, breast tenderness and cramping associated with PMS and joint pain in patients with rheumatoid arthritis. It's also been used to treat numbness and tingling in diabetics, skin problems like eczema, acne and rosacea, and patients with high cholesterol, multiple sclerosis and osteoporosis [source: Mayo Clinic].
The benefit of taking evening primrose oil is increasing the levels of GLA in your body, which could be low if you're not eating enough of the right fats, drink too much alcohol, have low thyroid function, or are undergoing radiation treatment [source: Brown].
But whether it will make your pimples or hot flashes go away remains unproven. In fact, scientific studies to support the claims that evening primrose oil helps cure any of these conditions are inconclusive. Most have been too small or poorly designed to produce reliable results, with one exception: eczema.
In a report by the Mayo Clinic, researchers gave evening primrose oil a "B" on a grading scale from A to F on the amount of evidence that supports claims it relieves itching, inflammation and flaking in patients with eczema [source: Mayo Clinic].
For most of the other conditions -- including multiple sclerosis, diabetes, breast pain, arthritis, high blood pressure and osteoporosis -- Mayo researchers gave the evening primrose oil a "C," or "unclear evidence" that it has any impact. For treatment of PMS and menopause, primrose oil got a "D" for lack of proof of its effectiveness [source: Mayo Clinic].
But for the same reason evening primrose oil may not help you, it probably won't hurt you either. Read on to find out why.