The veterinarian at The Animal Clinic, who put down a seven month- old mongrel deemed to be aggressive, had followed all protocol, said the Agri-Food and Veterinary Authority (AVA).
This included "necessary professional assessment" as well as deciding that euthanasia is a "reasonable option considering the circumstances".
In a statement on Tuesday, it said that the other vets at the clinic who examined the dog also found its behaviour to be hostile, and in subsequent visits its aggressiveness escalated.
This escalated to the point where it could not be muzzled by staff and "needed sedation for procedures that required more handling", said AVA.
The dog was eventually put down on Oct 7.
The move drew angry responses from the online community, saying adopter Ms Alison McElwee should have returned the puppy to the rehomer, before putting it down.
The AVA said that alternative measures were considered by Ms McElwee, including attempts to rehome the puppy.
Get The New Paper for more stories.