Fear of pain is one of the major reasons why patients fail to seek help from a dentist until their emergency is so severe that they are literally driven to seek professional help! Dental pain tends to be severe. It is not an uncommon sight to see a patient visiting a dentist with a glass of cold water in hand.

Generally, the anesthesia works very well with just one standard shot.  This is especially true if you are not already in pain when you come to the office. On the other hand, inflamed tissue (hot, red, swollen and painful) tends to be acidic in nature. The anesthesia is very PH sensitive. Anesthesia in a normal acid/base environment likes to seep into nerve fibers slowly, which is why anesthetics take some time to set under normal conditions. However, in an acid environment, the nerve fibers look to the anesthesia molecules like they are coated with wax and thus diffusion into the fibers is very slow. This is one of the reason the dentists usually advice antibiotics before starting any treatment.

Referred pain

Tooth related pain can be rather hard to understand.  Pain emanating from one tooth may be felt in another tooth far removed from the actual culprit.  This is why it is sometimes difficult for a dentist to make an accurate decision of which tooth to treat, especially if the tooth that the patient believes is the one that needs treatment shows no actual signs of disease.

When a nerve in a tooth becomes inflamed, the acid/base balance of the nerve tissue (which includes blood vessels and connective tissue as well as nerve axons) becomes acidic.  This acidity may be transferred along the nerve bundles via the blood vessels and connective tissue for quite a distance as it proceeds from the tooth toward the ganglion.  The acidity may be prone to diffuse throughout parts of the main nerve trunk and cause sympathetic pain in adjacent nerve bundles.  If the local inflammation is severe enough, it may even cause inflammation in entirely different branches of the Trigeminal nerve. Think of this as an electrical short circuit.  Severe inflammation in a nerve inside a tooth may cause a "short circuit" which  could "refer" the pain to any other branch of the Trigeminal nerve.  Thus pain in a back lower tooth may be felt in the ear, eye, or in an entirely different tooth.