User blog comment:Roy1313/Dimaria's Power/@comment-25111584-20160211163014/@comment-1776824-20160212201340

Using Dragon Slayer Magic comes at the cost of your humanity. You cease to be completely human. Look at Acnologia: he used his magic too much and eventually became a dragon. Just because he gets the benefits of being a dragon does not necessarily mean that the entire process leading up to it is harmful, otherwise Igneel and the others wouldn't have bothered to create antibodies.

Regarding Gray? The demonification process originally was implied to have been harmful, because Future Rogue described Frosch as being deliberately killed by Gray. That timeline clearly played out almost identical to the one we viewed in the main story. The only difference here is that Gray sought help from Porlyusica for his affliction. Gray's inability to properly wield the magic is entirely the reason why it was thought to be harmful in the first place.