it is certainly NOT normal practice to precribe and then keep it for other peoples use.
Doctors surgeries are highly profitable organsations and it is fundamentally wrong that this is happening.
I would do as chinadog has said and challenge this with the surgery.
Prescriptions are expensive, and the property issued belongs to you NOT the doctors, If you get free prescriptions, the tax payer is coughing up for the surgeries basic medical supplies which is very very wrong.