Local anesthesia is very safe, and there are few side effects associated with this medication. In rare cases, a swelling filled with blood (hematoma) may form if the needle hits a blood vessel. In other cases, the anesthetic may cause numbness in areas outside the region where the work is performed. For example, patients may experience drooping in the eyelids or corner of the mouth. This is due to the anesthetic spreading to and affecting the nerves that control these muscles. This can be disturbing to patients but the condition generally goes away when the local anesthetic wears off.

Though very rare, some patients may experience an allergic reaction to local anesthetics which is manifested as itching or rashes. The allergy is usually not to the local anesthetic itself but to the preservatives that are added to it. Patients should alert their dentist should this occur.

The vasoconstrictor in anesthetic solution may cause the heart to beat faster for a minute or two. The needle used to inject the solution can also injure a nerve, causing extended numbness and pain that usually heals over time.

Topical anesthetics also have very few risks. Some patients may have a mild allergic reaction that may cause swelling and raised welts on the skin that itch or burn. Severe allergic reactions are rare. It may be a day or two before the allergic reaction occurs.

Using excessive amounts of topical and local anesthetic can be toxic. Symptoms of toxicity include:

• Blurred vision
• Breathing difficulties
• Dizziness or lightheadedness
• Drowsiness
• Headache
• Ringing in the ears
• Seizures
• Shivering

Excessive amounts of topical anesthetic can also cause methemoglobinemia. This rare condition occurs when hemoglobin (a substance in red blood cells) is converted to an inactive form that fails to carry oxygen. In mild cases, the patient experiences no symptoms. More severe cases may be marked by fatigue, bluish or graying skin color, and breathing difficulties. The condition is treated with methylene blue which converts the inactive hemoglobin back to a normal functioning form.

Patients who have general anesthesia may feel nauseated afterward and may vomit. Other side effects include dizziness, drowsiness, headache, muscle aches, shivering, sore throat, vision problems and weakness. Most of these side effects disappear within 24 hours, although they can last for a few days.