Board Thread:Naruto Discussions/@comment-3965545-20160202201023/@comment-142748-20160203074715

QuakingStar wrote: There is literally no reason at all other than the disease that his chakra levels would be lower than Kakashi's who's transplanted Sharingan is the reason for low chakra in his case. Itachi was apparently taking medicine for years so if that was more specific we would know exactly how many years. It was never said that Itachi was taking medicine for years, QuakingStars. All we know he was taking medicine to prolong his life so he could fight Sasuke. That's it.

And its not unheard of for people to be born with naturally low stamina. Itachi has naturally low reserves despite the quality of his chakra. He had to compensate with better chakra control and his mastery of genjutsu.