What is mouthing Behaviour?
Mouthing behaviour in dogs is when a puppy (or adult) uses its mouth to during play. This is a perfectly natural thing for a puppy to do and is actually a very important aspect of their development. The most important thing pups learn from mouthing is bite inhibition, put simply, how hard they can bite during play.
They should learn this at a very early age whilst playing with their litter mates and one of the reasons puppies have such sharp teeth is to help learn bite inhibition.
The way pups learn bite inhibition is simple; they bite their siblings during play fighting which is completely normal, but, when they bite too hard their sharp teeth cause their playmate (who received the bite) to squeal and maybe freeze or walk off effectively ending the game.
The lesson is, if you bite too hard the game will end and nobody will play with you which, to a puppy, is a big consequence! The puppy should soon realise that biting during play isn’t acceptable but pretending to bite with no real pressure is fine and keeps the game going! That is successful bite inhibition.
Problem Mouthing Behaviour
Problems arise when the puppy begins to play with humans before it has started to learn bite inhibition and bites too hard whilst playing. Sharp puppy teeth combined with a forceful bite can really hurt, or worse cause injury!
As soon as a puppy is taken away from it’s litter mates and brought into their new family the bite inhibition lessons must continue! It is too easy to let the cute puppy chew on your fingers, hands, ears etc but every time you allow a puppy to bite you, however hard, you are effectively teaching it to bite.
A puppy biting is one thing but if they grow up to be an adult dog who doesn’t understand how hard to bite during play then you have a serious issue to deal with. Fortunately the best time to teach a dog not to bite or mouth is when they are a pup.
I personally don’t like dogs to use their teeth at all when playing with people. Humans, and especially children, are much more delicate than dogs and we are easily injured.
Guiding your dog through mouthing Behaviour issues
The easiest way to stop mouthing is to teach a puppy the same lessons they learn from litter mates; when bitten, act hurt and end the game!
In practice this means; as soon as the puppy’s teeth touch your skin, squeal, quickly withdraw your hand whilst turning away from your dog and freeze. It is vital during this reaction that you don’t talk to, look at or try to touch your dog, if you do the dog may think you are still playing.
What you have just communicated is;
1) that bite hurt and 2) I don’t want to play if you do that. After several seconds slowly turn back to you pup and offer to play again. If during the game your pup let their teeth touch your skin, squeal, withdraw, turn and freeze all over again. After a few repetitions the pup should begin to learn that in order to play with humans there can be no biting!

Offering an appropriate chew toy is a good idea, especially with pups or young dogs. Any mouthing should still be addressed but then offering a chew toy and rewarding (either vocally or with play) this action also teaches the dog what they should and shouldn’t chew.
Sometimes the mouthing is more ingrained which can often mean simply acting hurt isn’t enough to deter the dog from mouthing.
This is not an uncommon issue with rescue dogs, especially young ones If this is the case the consequence for mouthing must be increased, this is done by acting hurt (as before) but then also immediately leaving the room.
This communicates to the dog that if they do mouth during play, not only does the game end, but the all social contact is withdrawn. This should only be done for a minute or less, firstly; because withdrawing social interaction is quite a big consequence for a dog and secondly; to give the dog a chance to play without mouthing.
More Serious Cases of Mouthing
In more serious cases mouthing behaviour goes beyond unskilled play to a more forceful action often used to control a situation. It is this type of mouthing which is difficult to cure by acting hurt or by short time-outs, for this type of mouthing then an aversive sound may be used to disrupt/disagree with the mouthing.

Basically an annoying sound is made each time the mouthing behaviour occurs; the idea is the dog associates the annoying sound with the mouthing so stops mouthing in order to stop the noise. The required noise can range from a loud firm “Hey!”, Mikki disks, corrector air spray (not towards the dog) to simply or a rattle tin.
The sound must only serve to irritate the dog enough to distract the dog and stop it mouthing. Using a sound to scare the dog brings with it a new set of problems, therefore the sound mustn’t be so harsh the dog runs away or reacts defensively.
The sound should simply be enough to stop the mouthing behaviour and the play. As soon as the mouthing stops so should the sound, as soon as the mouthing starts so should the sound, this way it becomes very clear to the dog what causes the annoying noise.
It is always best to link an aversive stimulus with a command so only after a few exposures to the command then the loud stimulus the command alone should be enough to stop the mouthing.
I rarely use such aversives and do recommend you get a professional in before attempting such things. Mistiming such consequences or using them incorrectly can really undermine any trust your dog might have in you.