A new report says two thirds of mental health service users living in the community do not know how to make a complaint about their care.
Six out of 10 also feel disconnected from the community where they live, according to findings contained in a new report from Mental Health Reform.
The organisation says all service users should have an advocate who is appointed to fight their corner.
Director of Mental Health Reform Shari McDaid says many service users have difficulties getting what they need.
She said: "About a fifth said they were not involved at all in determining their own treatment and a significvant proportion also said they had had some difficulty in accessing various public services.
"A quarter have said they had difficulty in getting the mental health services they wanted or in getting housing services or in getting social welfare."