Evening primrose oil does not cause weight gain. Clinical trials involving obese women taking the supplement over 12 weeks found no significant difference in weight between those taking evening primrose oil and those taking a placebo. The supplement itself contains about 15 calories per 1,300 mg softgel, which is too small an amount to meaningfully affect your daily calorie intake.
What the Clinical Evidence Shows
The most direct evidence comes from a double-blind 12-week study of 100 women with substantial obesity. Researchers compared weight changes in women taking evening primrose oil against those taking a placebo. There was no significant difference in weight loss or gain between the two groups, whether participants were new to weight management or had a long history of struggling with obesity. The study’s conclusion was blunt: any effect evening primrose oil has on body weight is “clinically insignificant.”
A separate study actually found that gamma-linolenic acid (GLA), the key fatty acid in evening primrose oil, reduced weight regain in people who had recently lost a significant amount of weight. The researchers suggested GLA may play a role in how the body partitions fuel, potentially nudging it toward burning rather than storing calories. This doesn’t mean evening primrose oil is a weight loss supplement, but it does further undermine the idea that it promotes weight gain.
Why Some People Worry About Weight Gain
Evening primrose oil is a fat-based supplement, so it’s natural to wonder whether taking it daily adds up. A standard 1,300 mg softgel contains about 1.5 grams of fat and 15 calories. Even if you take two or three softgels a day (common dosing ranges from 500 mg to 6,000 mg depending on the condition), you’re looking at roughly 15 to 70 extra calories. For context, that’s less than a tablespoon of peanut butter. It’s not enough to shift your weight in either direction.
Some people also confuse bloating with weight gain. The most common side effects of evening primrose oil are gastrointestinal symptoms like abdominal pain, nausea, or diarrhea, according to the National Institutes of Health. Bloating or digestive discomfort can make you feel heavier or puffier without any actual change in body fat. If you notice your stomach feels unsettled after starting the supplement, that’s a GI reaction, not fat accumulation.
How Evening Primrose Oil Affects Metabolism
Evening primrose oil is rich in GLA, an omega-6 fatty acid that the body uses to produce anti-inflammatory compounds. GLA’s effects on metabolism are modest but lean in a favorable direction. Animal and preliminary human research suggests that evening primrose oil may improve insulin sensitivity, help regulate blood sugar, and support healthier cholesterol levels. In studies on diabetic rats, EPO supplementation restored more normal blood glucose patterns and increased insulin output from the pancreas. A separate finding noted that evening primrose oil combined with fish oil reduced fasting blood sugar, cholesterol, and body weight in people with type 2 diabetes.
None of this means evening primrose oil is a metabolic booster or a tool for weight loss. But the biological picture is consistent: nothing about how this supplement works in the body promotes fat storage or weight gain. If anything, GLA’s effects on inflammation and blood sugar regulation point in the opposite direction.
Typical Doses and What to Expect
How much evening primrose oil people take depends on why they’re using it. For PMS symptoms, clinical trials have used 1,500 mg to 6,000 mg per day. For menopausal hot flashes, studies have tested doses as low as 500 mg daily. Most over-the-counter softgels come in 500 mg or 1,300 mg sizes, so a typical routine involves one to four capsules a day.
At these doses, the caloric contribution is negligible. The supplement is well tolerated by most people, with digestive upset being the main complaint. Weight change, whether gain or loss, has not emerged as a side effect in any controlled trial. If you’re gaining weight while taking evening primrose oil, the cause is almost certainly something else in your diet, activity level, or hormonal health, not the supplement itself.