Officers arrived at Arthur Millwood Court, Salford at 17:40 BST on Monday following reports of a man falling from a balcony.
They found the 29-year-old with serious injuries next to the tower block on Rodney Street.
Police said one line of inquiry is examining whether he had taken a legal high known as Spice.
A man who fell from the sixth floor of a block of flats may have taken a legal high, said police.