Hi,
Evening primrose oil is a common supplement advocated for treating eczema. However, robust scientific data is lacking and in real life, many eczema sufferers derive no benefits from taking this or any other supplements.
This seems to be what my patients tell me as well. The key preventive measures lie with the avoidance of known aggravating factors such as stress, heat, sweating, friction, etc and to diligently repair/maintain the skin barrier using mild cleansers and regular use of moisturizers.
Judicious use of low-mid potency steroid creams under your doctor's supervision will treat eczema flares should they occur.
Hope this helps!