Having a tooth extracted or receiving a dental filling does not, in itself, invalidate the fast. However, the administration of an injection prior to such procedures—though subject to juristic disagreement—may invalidate the fast.
Moreover, during tooth extraction or filling, bleeding commonly occurs, and the blood may sometimes reach the throat. If the blood emerging from between the teeth is minimal and becomes indistinguishable within the saliva, swallowing it does not invalidate the fast. However, if the blood predominates over the saliva, such that it clearly outweighs it, then swallowing it invalidates the fast. In this case, only qaḍāʾ (making up the day) is required; kaffārah (expiation) is not.
Additionally, such medical procedures may cause weakness and potentially endanger the fasting person’s health. For this reason, it is preferable to postpone these treatments until after breaking the fast. If the situation is urgent or medically necessary, one may refrain from fasting on that day and make it up after Ramadan.