The provision of universal health care by the Portuguese NHS depends on the allocated government budget to health. Several reforms have been implemented over the last decades to improve access while ensuring the financial sustainability of the health care system. However, a practical and useable definition of public health sustainability is hard to find. We show that under two alternative definitions – both related to fiscal space and compliance with sound public finances – public health spending increase is limited. Our analysis indicates that public health spending growth levels below 3% can be financially sustainable. Taking into account that financial sustainability is a function of economic growth and will depend on the level of control of other public spending, our forecast for long-run health spending growth is compatible with the financial sustainability targets defined.