Evening primrose, Oenothera biennis, is a wildflower native to North America whose pale yellow flowers open in the evening. Its small seeds are pressed to make evening primrose oil, which is unusual because it contains gamma-linolenic acid, or GLA, an omega-6 fat that is rare in everyday foods.
The oil typically contains around 8 to 10 per cent GLA, alongside a larger amount of linoleic acid. Other sources of GLA include borage (starflower) oil and blackcurrant seed oil. The body can make GLA itself from linoleic acid, then uses it to produce signalling molecules involved in many normal processes.
Evening primrose oil has traditionally been used by women for comfort through the monthly cycle, and it has a long history of use for skin. In the UK it was once a licensed medicine for eczema and breast pain, but those licences were withdrawn in 2002 because evidence did not support the benefit, not because of safety concerns. Today it is sold as a food supplement, usually in softgels.