A neurology review sparked by concerns about a consultant's work has cost around £1 million so far, Northern Ireland's department of health said.
A total of 3,544 former patients of Dr Michael Watt have been or are in the process of being recalled. Many have epilepsy.
Patients who were being actively treated by Dr Watt have already been seen.
A total of 1,044 who had been seen by the consultant but discharged to the care of their GP and prescribed medications for neurological conditions will be offered appointments within the next four months, Stormont's health authorities said.
The medications include anti-epileptic drugs, immunosuppressants and disease modifying therapies which are used to treat conditions like epilepsy and Multiple Sclerosis.
Patients will be drawn from the NHS and two private clinics where Dr Watt practised.
Northern Ireland's chief medical officer Dr Michael McBride (pictured) said: "We want to apologise for any distress and uncertainty that this process brings.
"It is clearly our responsibility to act on clinical advice and the emerging outcomes from the first phase of the recall."
The decision on who to offer appointments to has been shared with the Royal College of Physicians, whose report triggered the review.
It described the action as "very sensible" and in line with its initial findings.
Additional clinics have been secured from the independent sector to support consultants at Northern Ireland's largest public healthcare provider, the Belfast Health and Social Care Trust.
All patients in the first stage of the review, announced earlier this year, have already received a first appointment and more than 600 discharged.
A significant number of other patients have been referred separately from the main recall process.
Belfast Health and Social Care Trust consultant neurologist Dr John Craig said: "We wish to reassure everyone who has received a letter that our primary focus is to provide assurance that they are on the correct treatment and request that they should not stop or make adjustments to their medication until they have been reviewed by a consultant neurologist at their appointment.
"It is crucial that everyone who does not receive a letter understands that they have no action to take and do not need to get in touch.
"This latest review process is being concentrated on specific groups of patients taking specific, specialised medicines."
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