So... I found this in Reddit.

Q: Are there any rules that determine the appearance of an Ishgardian who transforms into a dragon?
It is determined by the strength of the presence of the dragon’s blood within them. Diresaur (JP: Were-Dragon) is the strongest, followed by Syricta and then Aevis.
In addition, while they say that Ishgardians will transform upon drinking “dragon blood”, it actually has to be “blood imbued with magical power” in order for anything to happen. Therefore, “Draconian Potions” made with the item “Dragon Blood” are safe to drink.
As long as you stick to the recommended dosages you’ll be fine, so players whose characters are meant to be of Ishgardian descent can use draconian potions without worry.
Source: some QA I don't really remember.