Were the seeds of the cocaine habit placed in
A Study in Scarlet? One could argue fom the folllowing scene that Holmes was subject to some mental malady, perhaps bi-polar disorder.
...Nothing could exceed his energy when the working fit was upon him; but now and again a reaction would seize him, and for days on end he would lie upon the sofa in the sitting-room, hardly uttering a word or moving a muscle from morning to night. On these occasions I have noticed such a dreamy, vacant expression in his eyes, that I might have suspected him of being addicted to the use of some narcotic, had not the temperance and cleanliness of his whole life forbidden such a notion.