A hematoma is a localized mass of extravasated blood, usually clotted, that is relatively or completely confined within an organ or tissue. Hematomas differ from smaller bruises in that the latter occur as a result of damage or injury to small blood vessels while the former comes from damage to larger blood vessels. A hematoma can occur in dentistry following a surgery, treatment, or other oral procedure where the tissues of the mouth are manipulated in such a way that blood vessel damage can occur. Though hematomas are generally not dangerous and can be a common side effect of some dental procedures, the practitioner will often examine it to ensure it will heal without additional aid. Some patients experience a hematoma following an injection used to numb or block the sensation in the tissues around a dental procedure site. Most hematomas will resolve on their own.