User blog comment:Sargenttacos115/The side efx of Dragon slayer Magic/@comment-4829046-20131109150759/@comment-7418318-20131109152359

It was talked about in Atlas Flames memories of Igneel.

He commented how Natsu eating his flames felt the same as when Igneel did it.

As for other side-effect that some of the Dragon Slayers show, which may or may not be due to them being Dragon Slayers:

Slit pupils, odd eye colours, short temper, attractinc Exceeds, resistance to certain kinds of magic (their own element, regular Anima), possible vulnerbility to certain kinds of magic (Freed's rune magic).