Both flaxseed oil and evening primrose oil can be healthy parts of your diet, and you can consume both at the same time. Ingesting more flaxseed oil or flaxseeds than evening primrose oil can help balance your intake of omega-3 fatty acids and omega-6 fatty acids. Possible side effects related to flaxseed oil are diarrhea if you consume large amounts and the increased risk of bleeding if you suffer from a bleeding disorder. Nausea, upset stomach or diarrhea may occur if you take too much evening primrose oil.