Corrected entry: When Mia is first possessed she manages to walk around and shoot at her brother while drugged up on enough tranquillizer to 'put a horse to sleep', but at the end of the film it only takes one shot of the tranquillizer to knock her out for long enough for him to not only bury her, but give his teacher friend a fond farewell too.
Corrected entry: When David is burying Mia, she speaks with no impairment despite her tongue having been split lengthwise in a previous scene.
Correction: She's still possessed by a demon at this point. Throughout the film she's been doing all sorts of things that ought to be physically impossible for a mortal human to do.
Correction: It was a huge dose of tranquilizer she was given at the end; the line about putting a horse to sleep was pure hyperbole. The fact that it was delivered in a single shot is irrelevant given the sheer size of the syringe's barrel.