Evening Primrose oil has been used to treat arthritic pains, Pre-menstrual tension and menopause by herbal practitioners from medieval times. There have been many scientific trials in recent years to attempt to elicit this claim.
However despite anecdotal evidence none of the trials showed that they had any proven benefit. It is likely that as PMS and menopause have variable symptoms there is a large degree of placebo effect in helping hot flashes or breast pain. It does not show any alteration in oestrogen or testosterone in test subjects and should not be relied to do this.
As evening primrose comes under the category of supplemental agent it is not governed by the same stringent guidance and approval for drugs so manufacturers are free to claim that it has more benefits than it does and sell large quantities to the unsuspecting public.
In ancient times the extract of evening primrose was called 'kings cureall' where many believed it helped a lot of diseases including eczema,heart, arthritis, pregnancy problems, PMS, menopause, breast pain etc. it is likely these beliefs stuck and as it is cheap to produce and can be sold at a profit ... There may be some benefits to taking Gamoleneic acid but definitely not raise testosterone based on current evidence.