Section 47JJ. Any policy, contract, agreement, plan or certificate of insurance issued, delivered or renewed within the commonwealth, which is considered creditable coverage under section 1 of chapter 111M, shall provide, for any covered drug that is a narcotic substance contained in schedule II pursuant to chapter 94C and subject to cost sharing, that if a person receives a prescription filled in a lesser quantity pursuant to section 18 of said chapter 94C, said person shall not be subject to an additional payment obligation, including, but not limited to, co-payments, if said person fills the remaining portion of the prescription.