By including a plurality of different groups it is possible to maintain the therapeutic level of the drug over a long period of time e.g. 1 day, several days (2 to 6 days) to 1 week, and even several weeks (2 to 3 weeks) to 1 month. [0153] Those skilled in the art will recognize that variability in terms of the rate at which the coating material dissolves can be changed by increasing the thickness