I have a water dragon with a good size lump forming on his top jaw does anyone know what before going to vet?

Answers:
Reptiles can get mouth rot.

GO TO THE VET!

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Could be an abcess- An infection under the skin that fills the area with pus. They are pretty common and treatable.
Could be Metabolic Bone Disease. Are you feeding him correctly and giving him enough heat and UV light? Treatable, but can be difficult.
A vet will tell you more, Good Luck.

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go to the vet it could be an infection

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You need to take it to the vet for the correct answer, it coulb be an abcess or MBD (metabollic bone disease). The best thing for your water dragon is to be diagnosed by a reptile vet. Good luck and take care of that baby!