There are two ways of proceeding. If there is a medical reason to use a more expensive drug, there is currently an override system so that it can be refunded.
Suppose there is a generic version of the drug, but the patient wants to continue using the brand-name drug because of the colour, taste or something like that. If it isn't demonstrated that, medically, the brand-name drug provides additional therapeutic benefits, the person should still be able to get it, but that person, not taxpayers, should assume the difference in cost.