From Inane Scanlation Ch. 257 pg. 8
Kakashi: Besides the fact that it requires a considerable amount of chakra, it also presents a great risk to your eyes as well doesn't it... Itachi.
Nevermind the bad punctuation. This right here says it hurts your eyes.

From Inane Scanlation Ch. 257 pg. 9
Kakashi: How far has... "plink" your eyesight deteriorated?
NOT A DAMN METAPHOR!!!