EPO would be one. It increases oxygen supply to the muscles, enabling players to run faster, further and for longer. One study of the effects of EPO showed a 54% increase in performance within four weeks of starting it.
Now think about that and think about a team that was, on all indicators, one of the least fit last season and is suddenly, on those same indicators, one of the most fit. And it wasn't gradual, it was virtually overnight. and from escaping relegation by the skin of their teeth,they're about to win the league. As much as I like Ranieri, there is simply no other rational explanation.