19-year-old Jack Hester-Wox did not return to Cheswold Park Hospital following an unescorted visit to a local beauty spot. He was reported missing at 5.30pm on Thursday 10 October 2013.
Police believe he may be in the York area and are urging anyone who knows of his whereabouts to contact the police as a matter of urgency.
Officers are currently carrying out searches in the York area, but are asking for assistance from members of the public to report any sightings.
Any members of the public who see Mr Hester-Wox should not approach him but call the police immediately on 101.
Superintendent Phil Cain of York Safer Neighbourhood Team, said:
It’s vital that Mr Hester-Wox is returned to the hospital to complete his treatment.
We believe he may have travelled to York and are appealing for anyone who has seen him since 2pm on Thursday 10 October or knows of his current whereabouts to please contact the police.
Please do not approach him but all the police on 101.
In an emergency always dial 999.
Mr Hester-Wox is described as white, 6ft tall, of large build, he has blue eyes and light brown to fair hair.