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. Analgesics are typically used in conjunction with anesthetics to control pre-, intra-, and postoperative pain.

Local anesthetic agents prevent the transmission of nerve impulses without causing unconsciousness. They act by reversibly binding to fast sodium channels from within nerve fibers, thereby preventing sodium from entering the fibres, stabilising the cell membrane and preventing action potential propagation. Each of the local anesthetics has the suffix "–caine" in their names.

Local anesthetics can be either ester- or amide-based. Ester local anesthetics are generally unstable in solution and fast-acting, are rapidly metabolised by cholinesterases in the blood plasma and liver, and more commonly induce allergic reactions. Amide local anesthetics are generally heat-stable, with a long shelf life (around two years). Amides have a slower onset and longer half-life than ester anesthetics, and are usually racemic mixtures, with the exception of levobupivacaine (which is S(-) -bupivacaine) and ropivacaine (S(-)-ropivacaine). Although general rules exist for onset and duration of anesthesia between ester- or amide-based local anesthetics, these are properties are ultimately dependent on myriad factors including the lipid solubility of the agent, the concentration of the solution, and the pK_a. Amides are generally used within regional and epidural or spinal techniques, due to their longer duration of action, which provides adequate analgesia for surgery, labor, and symptomatic relief. Some esters, such as benzocaine and tetracaine, are found in topical formulations to be absorbed through the skin.

Only preservative-free local anesthetic agents may be injected intrathecally.

Pethidine also has local anesthetic properties, in addition to its opioid effects.

Volatile agents are typically organic liquids that evaporate readily. They are given by inhalation for induction or maintenance of general anesthesia. Nitrous oxide and xenon are gases, so they are not considered volatile agents. The ideal volatile anesthetic should be non-flammable, non-explosive, and lipid-soluble. It should possess low blood gas solubility, have no end-organ (heart, liver, kidney) toxicity or side-effects, should not be metabolized, and should not irritate the respiratory pathways.^{[citation needed]}

No anaesthetic agent currently in use meets all these requirements, nor can any anaesthetic agent be considered completely safe. There are inherent risks and drug interactions that are specific to each and every patient. 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.