National authorities can introduce a system of "prior authorisation" for going to another Member State for treatment in 3 cases:
- For healthcare which involves overnight hospital stay of at least one night
- For highly specialised and cost-intensive healthcare
- In serious and specific cases relating to the quality or safety of the care provided by the particular provider in question
In these three cases, patients may need to ask for permission in advance from their national health authority in charge of reimbursement. Member States are required to set out publicly which treatments are subject to such authorisation – you can find the list via your National Contact Point. (Source: EU Commission MEMO/11/32.)