Abstract Background Proton pump inhibitors (PPI) are frequently overprescribed, posing challenges to patients and healthcare systems. In Portugal, the publicly funded National Health Service (NHS) provides universal coverage and reimburses the cost of medications regardless of the prescription origin. If private practices may benefit from longer consultation times allowing better prescription, they do not face any incentive to avoid over-prescription. We aimed to compare outpatient prescription trends, patterns, and costs of PPI among older adults in private and public sectors. Methods Nationwide retrospective ecological study on PPI prescribed for older adults in Portugal 2020-2022. Data on defined daily doses (DDD) and costs were obtained from a national public database by healthcare sector, sex (female/male), and age group (65-74, ≥75). We analysed trends, market share of DDD per 1000 older adults per day (DID), and mean cost per DDD (€/DDD) for all PPI substances. Results We observed a decrease in prescribed PPI from 349.0 DID in 2020 to 341.9 DID in 2022. Omeprazole (reference and cheaper substance) was the most prescribed drug for females and pantoprazole for males in the public sector. Similar pattern was observed among those aged ≥75 in the private sector while for those aged 65-74, esomeprazole was the most prescribed drug. Overall, the private sector prescribed PPI with a 20% higher price (0.126 €/DDD) than the public (0.106 €/DDD), with greater differences among the most expensive substances (rabeprazole +0.032 €/DDD; lansoprazole +0.029 €/DDD; esomeprazole +0.021 €/DDD). Conclusions PPI prescription followed similar trends in both sectors, but private practitioners tend to prescribe more expensive drugs, because their patients can afford them and because there is no incentive for more rational prescription. Since the NHS financially supports drugs prescribed in private practices, financial incentives towards rationale prescription should also fall in this sector. Key messages • Overall, the private sector prescribed PPI with a 20% higher price (0.126 €/DDD) than the public (0.106 €/DDD), with greater differences among the most expensive substances. • Since the National Health Service financially supports drugs prescribed in private practices, financial incentives towards rationale prescription should also fall in this sector.