Jordan Thomas, 17, who was attending ongoing treatment was last seen near the main entrance of the hospital around midday, according to the Metropolitan Police.
He is described as a black male, around 5ft8ins, of slim build, and was last seen wearing a black hooded top, blue jeans and black ‘Nike’ shoes.
Police in Westminster are appealing for help to trace the youngster and are growing increasingly concerned for his welfare.
Jordan is believed to have links to the Southwark and Merton areas.
Officers are keen to hear from Jordan himself, or anyone who knows of his wherabouts.
Anyone with information should call Westminster police on 07500 766 462 or police via 101 or alternatively the charity Missing People on 116 000.