An anesthetic (or anaesthetic) is a drug to prevent pain during surgery. A wide variety of drugs are used in modern anesthetic practice. Many are rarely used outside anesthesia, although others are used commonly by all disciplines. Anesthetics are categorized into two classes: general anesthetics, which cause a reversible loss of consciousness, and local anesthetics, which cause a reversible loss of sensation for a limited region of the body while maintaining consciousness. Combinations of anesthetics are sometimes used for their synergistic and additive therapeutic effects. Adverse effects, however, may also be increased.