Esmeron® is one of a group of drugs called muscle relaxants. Muscle relaxants are used during an operation as part of the general anaesthetic. When you have an operation your muscles must be completely relaxed. This makes it easier for the surgeon to perform the operation. Normally the nerves send messages to the muscles by impulses. Esmeron® acts by blocking these impulses so that the muscles are relaxed. Because the muscles needed for breathing also become relaxed you will need help with your breathing (artificial respiration) during and after your operation until you can breathe on your own. During the operation the effect of the muscle relaxant is constantly checked and if necessary some more drug is given. At the end of surgery the effects of the drug are allowed to wear off and you can start breathing on you own. Sometimes another drug is given to help speed this up. Esmeron® may also be used in Intensive Care Units to keep your muscles relaxed.