Children and young people accessing child mental health services with open cases have been "lost" to follow-up care.
That is the finding of a new Inspector of Mental Health Service report, which shows in one community health organisation there were 140 "lost" cases within the Child and Adolescent Mental Health Service (Camhs) team.
There is evidence some teams were not monitoring antipsychotic medication, in accordance with international standards.
Mental Health Commission chief executive John Farrelly said this medication must be monitored.
"Medication is very helpful and needed for children, but when you put people on antipsychotic medication, particularly children, you need to follow up in terms of their physical health... because there can be side effects of the medication.
"Really what we want to do with every child on this medication is that their files are reviewed, and they get follow-ups to make sure that they are safe and well."
The report, led by Dr Susan Finnerty, also found children waiting days in emergency departments for psychiatric care, psychiatrists not trained adequately to deal with children, and overwhelmed and inadequately supervised staff.
HSE chief operations officer Damien McCallion said a “major improvement process” was under way at Camhs, involving “a senior clinical/operational team”.
“The HSE engaged with [Dr Finnerty]... and where specific concerns were identified, we immediately put in place targeted action plans to address them. In the case of all children where concerns have been raised... these have been managed directly by the service caring for them.”