system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Could a Metamorphmagus (i.e. Tonks) simply cure themselves of a serious wound?
gpt
From first glance, yes. They can "chang[e] sex and age, potentially looking like anyone at all, or even just changing a part of their appearances, such as the shape of their noses" according to Ministry Records (Harry Potter Wiki, which cites Order of the Phoenix). They should be able to heal wounds if they can do the stuff stated above, unless there's some other rule at play here.

We have one known Metamorphmagus death on Ministry records, that of Nymphadora Lupin nee Tonks. However, she appears to have died of the Killing Curse, as the records state that she was "pale and still and peaceful-looking, apparently asleep beneath 
the dark, enchanted ceiling" after her dead body was brought to the Great Hall of Hogwarts. 

She might not have died of the Killing Curse, however. If she had been hit with something causing a serious wound, she may have been only able to heal it on the surface, while leaving the injuries on the inside.

What can we conclude of this data? We can conclude that it is likely that Metamorphmagi can heal serious wounds, however there is no evidence to prove that it is possible, and shaky evidence to rebut this conclusion.