my boxer has a swollen mouth flap?

Took him to the vet and she gave him a steroid injection;how long will it take to work ,the vet said he may have been stung if not he could have something stuck in his mouth;if the swelling hasnt gone by morning he has to have an xray I am so worried,

Usually the swelling will go down in 72 hours.

Watch your dog carefully for any more swelling and listen carefully to his breathing as some dogs can have severe allergic reactions to bee stings.His possibly being stung in the flute (mouth flap) is a sensitive area which is of course near the airway. If he shows any signs of breathing difficulties or additional swelling call your vet, or if it happens during the night have an emergency vets number by the phone to call them immediately.Personally I would call an emergency vet and just give them a heads up so that in the event something happens they will be prepared when you get your dog to their clinic.