Mass. Gen. Laws ch. 118E, § 10L

The division and its contracted health insurers, health plans, health maintenance organizations, behavioral health management firms and third party administrators under contract to a Medicaid managed care organization or primary care clinician plan 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.