Local anesthesia, in a strict sense, is anesthesia of a small part of the body such as a tooth or an area of skin. Common techniques include topical anesthesia (surface), and local infiltration.
Adverse effects depend on the local anesthetic agent, method, and site of administration and is discussed in depth in the local anesthetic sub-article, but overall, adverse effects can be:
localized prolonged anesthesia or paresthesia due to infection, hematoma, excessive fluid pressure in a confined cavity, and severing of nerves & support tissue during injection.
systemic reactions such as depressed CNS syndrome, allergic reaction, vasovagal episode, and cyanosis due to local anesthetic toxicity.
lack of anesthetic effect due to infectious pus such as an abscess.