It has been considered most appropriate to assign fixed DDDs based on the average use of the different combinations without considering and comparing the strengths of the various components. One UD is the fixed DDD for products dosed as 1 tablet daily whereas two UD is the fixed DDD for products dosed as 2 tablets daily. The assigned DDDs cannot always be compared with the DDDs assigned for plain preparations.
See list of DDDs for combined products, www.whocc.no.