They are given before medical treatment, such as surgery, that would otherwise cause pain. There are many anesthetic drugs, and they can be grouped according to the effect they produce in the patient. A general anesthetic, causing loss of consciousness as well as loss of sensation, is given in most surgical operations. Another group, called local anesthetics, causes loss of sensation only in the area to be treated.
Dentists often use a local anesthetic during the filling or extraction of a tooth.
A third group is the topical anesthetics, which remove sensation from a surface area such as an eye or the nose. The anesthesiologist reviews the patient's medical history and may order tests to help select the anesthetic. Later, immediately before the general anesthetic, an injection of a short-period sleeping drug is given in a vein. This takes effect extremely quickly. The drugs that continue the general anesthesia are injected or given as gases either through a face mask or a tube inserted through the mouth or nose into the windpipe.