Members of the British Medical Association in Northern Ireland say patient care on both sides of the border will be at risk if a hard border is imposed after Britain leaves the EU.
The BMA is the professional association and trade union for doctors.
At the moment, there are a number of cross-border health services like the cancer centre in Londonderry.
All-island care has been expanded in areas such as children's cardiac services.
There's also long-standing co-operation between the emergency services when responding to major emergencies and public health risks.
The BMA's Northern Ireland council chair, Dr John Woods, said it's vital this work continues.
"Northern Ireland is too small a health economy to efficiently provide some smaller specialist services - the Republic of Ireland is our natural partner for many of these, allowing both countries to provide benefits to patients on both sides of the border," he said.
"Such projects mean that on a day-to-day basis doctors across Northern Ireland and the Republic will be in contact, getting professional advice, sharing knowledge and collaborating on patient care."
"Any border restrictions imposed after Brexit would risk reversing this progress and would damage patient care."
The BMA has said it wants mutual recognition of qualifications to continue, as well as the existing ability of doctors to move freely between the jurisdictions.
Dr Peter Maguire, a consultant anaesthetist who lives and works in Newry but also works regularly in Monaghan, said in the event of a hard Brexit, he will have "no choice" but to move south of the border.
"It's not just cross-border movement that will be an issue for patients - 54% of the exports from Ireland are pharmaceuticals and huge amounts of medical equipment are made and come from Ireland," he said.
"What will happen if there are tariffs? What will happen to the cost of drugs with the change in the euro?"
Another organisation which has concerns about what could happen after Brexit is Cooperation and Working Together (CAWT). It's a partnership between the health services in Northern Ireland and the Republic of Ireland.
Chief officer Bernie McCrory crosses the border 10 times on her daily commute to work.
"We've been guaranteed that EU funding for any project that has been approved before the day of Brexit will be underwritten by the exchequers in both jurisdictions for the duration of the project, so we're not concerned about the current state of projects which will sustain until 2021," she said.
"What happens beyond that is a matter of discussion."
However, she added that cross-border health care isn't fully dependent on EU funding and is convinced that the British and Irish governments will continue their commitment to the radiotherapy unit in Altnagelvin.
"There are lots of service models, if you take for example the Norwegian model where Norway is not in the EU it nonetheless benefits from EU funding because of its adjacency to other EU countries.
"There may be innovative ways to deliver projects which are not the same as we currently use."