Pharmaceutical reimbursement policies should aim for satisfactory health care at reasonable prices and assure treatment adherence while avoiding wasteful pharmaceutical spending. In Belgium, the maximum billing system, also called "Maximum Amount Fixed" system (MAF), ensures that out-of-pocket payments beyond a ceiling are fully reimbursed to guarantee pharmaceutical treatment in vulnerable population groups (e.g. low-income people and chronic patients). Starting from 2015, a policy change occurred and these expenses were waived during the same calendar year once this ceiling was reached instead of reimbursing these expenses in the next fiscal year. Each subsequent fiscal year, out-of-pocket payments were reintroduced in January. Longitudinal dispensing trends, from early 2013 to mid-2020, for 13 prevalent reimbursed opioids were investigated. For MAF patients, significant seasonal increases in opioid dispensing reoccurred towards the end of the year. This pattern was absent for non-MAF patients and was only observed after the 2015 policy change. Periodic changes in out-of-pocket expenses, a characteristic of the reimbursement policy in Belgium, clearly influences opioid availability in an already at-risk group. Out-of-pocket reimbursement policies should be urgently re-evaluated to minimize unnecessary opioid exposure, while preserving the affordability of pain treatment for vulnerable patients.