Use the company's grievance procedure. You could do this formally - with a written complaint and a meeting, or informally - with a meeting with your manager followed up by a written record of what was agreed at the meeting.
You'll have to be clear what your complaint is. The co-worker is affecting the productivity of the rest of the team? The co-worker is a disruptive influence? The co-worker is treated more leniently than other members of staff? The co-worker poses a danger to himself and other?
You'll also have to be clear what you want the manager to do. Have the manager more closely supervise the co-worker? Put the co-worker on sick leave? Help the co-worker get treatment? Have the co-worker disciplined? Give the co-worker tasks appropriate to his skills?
If you use the grievance procedure the company has a duty to take action. Since the grievance will be recorded it can be used in case of constructive dismissal.