Section 17P. The commission shall provide, to an active or retired employee of the commonwealth who is insured under the group insurance commission, 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, that person shall not be subject to an additional payment obligation, including but not limited to co-payments, if that person fills the remaining portion of the prescription.