A Study in Scarlet - a couple of pills were found in the room of Joseph Stangerson. When Holmes heard this he cried out 'The last link. My case is complete'.

 

The Sign of Four - Holmes would have called it a medicine however Watson had other ideas. In the first paragraph of the story Holmes picks up a bottle and hypodermic syringe and injected himself in his left arm, before releasing a 'long sigh of satisfaction'.

 

The Sign of Four - Holmes took a seven percent solution of cocaine at the start of the stories, much to Watson's disgust.

 

The Sign of Four - Watson is thought, by Holmes, to have cautioned Thaddeus Sholto "against the great danger of taking more than two drops of castor-oil, while I recommended strychnine in large doses as a sedative".