From catastrophic health expenditures to the cost of unmet needs: New metrics for pharmaceutical access in vulnerable populations.

Ensuring equitable access to essential medicines requires indicators that not only identify who faces unmet need but also quantify its budgetary implications. Traditional measures, such as catastrophic health expenditure (CHE), capture visible episodes of financial hardship, while self-reported unmet need is sensitive to perceptions and survey design; neither readily delivers estimates of how much it would cost to close affordability-related gaps. Using five waves of harmonised Portuguese Household Budget Survey (HBS) microdata (2000-2022/2023), we first describe trends in out-of-pocket spending on medicines and CHE by age group and consumption quintile, applying the WHO Europe 40% capacity-to-pay threshold. We then introduce an expenditure-gap method that compares observed medicine spending in the poorest quintile with that of less constrained groups within each age-year cell. The resulting gaps are interpreted as monetised unmet pharmaceutical need and summarised as a robustness range across alternative benchmarks and sample restrictions. Results show that vulnerability in access to medicines is most pronounced among older adults in low-consumption quintiles, who combine a high out-of-pocket burden, persistently elevated CHE incidence, and sizeable expenditure gaps relative to better-off peers. The framework converts under-consumption of medicines into euro-valued estimates of unmet need, offering policymakers an easily replicable tool to approximate the budgetary cost of closing affordability-driven gaps. Because household budget surveys are harmonised across European countries, the approach can be extended to other settings and health spending categories to monitor progress towards more equitable pharmaceutical coverage.