As the medication is priced the same for client whatever the dose they simply go along with that.

However, there is of course a cost to the system(NHS) that could be reduced if the GP only adjusted the dose so that the client could take grapefruit juice. Since this would mean varying the dose, say standard for client without grapefruit to lower with grapefruit, then it is easier to just say no grapefruit and not to care for the costs as the GP does not pay.

Sadly the taxpayer does pay and the grapefruit vender has a lost sale. His business go bust and he gets sick through the stress of being unable to feed his family. So his GP gives him medication and he must not have grapefruit which he loves………..urgh!