A poorly fitting bit is a common cause of mouth trauma. For instance, rubs in the corner of the mouth are possibly the most common form of such trauma and they may be caused by using a bit that is too small but perhaps also by a mouthpiece that is too wide. In this case the cause is friction as the bit slides back and forth as the rider asks for turns etc.
You are probably the only person that has the opportunity to check your horse’s mouth on a regular basis.
It is important to look for anything unusual, including any signs of bruising, cuts, etc. Check out the tongue and don’t forget to look underneath as ulcers and trauma can occur here. The tongue may be lifted gently in order to look underneath but do not grab it and hang on as it is a muscle and damage may occur. Check out the upper and lower palate and bars for any sign of rubbing. Don’t forget to look inside the cheeks, in case the flesh has been pushed into the teeth by the cheek of the bit. This is not an uncommon injury and is often only found by the dentist. Obviously if the external corners of the lips are rubbed it will be clearly apparent, but don’t forget to turn the corner out to check for rubbing inside.
It is also important to consider which type of bit is correct for your horse. If you are unsure, take a look through the different bits available from Neue Schule as a starting point.