thumb|right|Leaves of the [[coca plant (Erythroxylum novogranatense var. Novogranatense), from which cocaine, a naturally occurring local anesthetic, is derived]]
An anesthetic (American English) or anaesthetic (British English; see spelling differences) is a drug used to induce anesthesia — in other words, to result in a temporary loss of sensation or awareness. They may be divided into two broad classes: general anesthetics, which result in a reversible loss of consciousness, and local anesthetics, which cause a reversible loss of sensation for a limited region of the body without necessarily affecting consciousness.
A wide variety of drugs are used in modern anesthetic practice. Many are rarely used outside anesthesiology, but others are used commonly in various fields of healthcare. Combinations of anesthetics are sometimes used for their synergistic and additive therapeutic effects. Adverse effects, however, may also be increased. Anesthetics are distinct from analgesics, which block only sensation of painful stimuli.
General anesthetics
alt=Bottles of sevoflurane, isoflurane, enflurane and desflurane, the most common fluorinated ether (flurane) inhalation anesthetics.|thumb|Bottles of sevoflurane, isoflurane, enflurane and desflurane, the most common fluorinated ether (flurane) inhalation anesthetics. Fluranes are color-coded – sevoflurane is marked yellow, isoflurane purple, enflurane orange and desflurane blue. The notches visible below the bottle caps are unique to each agent, ensuring that a vaporizer can only be filled with the correct agent. The agents in widespread current use are [[isoflurane, desflurane, sevoflurane, and nitrous oxide. Nitrous oxide is a common adjuvant gas, making it one of the most long-lived drugs still in current use. Because of its low potency, it cannot produce anesthesia on its own but is frequently combined with other agents. Halothane, an agent introduced in the 1950s, has been almost completely replaced in modern anesthesia practice by newer agents because of its shortcomings. Partly because of its side effects, enflurane never gained widespread popularity. However, though they produce unconsciousness, they provide no analgesia (pain relief) and must be used with other agents. When it is used, it is often paired with a benzodiazepine such as midazolam for amnesia and sedation. So, while they are rarely used to induce anesthesia, they are frequently used along with other agents such as intravenous non-opioid anesthetics or inhalational anesthetics.