11.— Where a pharmacist who is working in a retail pharmacy business is presented with a prescription, by the patient for whom the prescription was issued or a person acting on behalf of the patient, for an interchangeable medicinal product under a common name, the pharmacist shall, for the purpose of that prescription, dispense from amongst the interchangeable medicinal products which fall within the group of interchangeable medicinal products concerned and which the retail pharmacy business has in stock at the time the prescription is so presented, that medicinal product which is of the lowest cost to the Executive (as specified in the Reimbursement List) or the patient, as the case may be.