Muscle relaxants are drugs used to relax, or decrease the tone of, skeletal muscles, those that are responsible for voluntary movement of the human body. There are different types, with different applications. Neuromuscular-blocking drugs completely block the transmission of nervous signals to the muscles, resulting in paralysis. Others, such as benzodiapines and methocarbamol, relax the muscles to a lesser degree and are used to manage seizures, muscle spasms, or pain associated with muscle injury.
Neuromuscular-blocking drugs are most often used during surgery to prevent complications resulting from involuntary muscle movement. These muscle relaxants can affect smooth muscle, such as the diaphragm, as well as skeletal muscle, so artificial respiration may be necessary. Neuromuscular-blocking drugs do not block the sensation of pain, so they must be used in conjunction with anesthetics.
Muscle relaxants taken to relieve pain or spasms are only available through prescription in the United States. Benzodiapines have other effects and uses, such as sedation and decreased anxiety, and they are sometimes used and abused recreationally as well as for medical concerns. Prescription medications work on the central nervous system, while neuromuscular-blocking drugs work directly on the muscles, making them insensible to neurotransmitters. Movement is typically possible, though impaired, under the effect of benzodiapines and similar drugs.
Prescription muscle relaxants have a number of side effects. Many types carry a high risk of dependence, and withdrawal can be very uncomfortable and even fatal. Some do not carry this risk, or have a less severe risk of dependence. A patient may also build a tolerance to the drugs, especially with heavy use.
Muscle relaxants cause impaired movement and coordination, as well as drowsiness, as you would expect, so one should not drive or operate machinery under their effect. Combining them with alcohol is extremely dangerous and can result in coma or death. For all of these reasons, it is important to take these medications only under the direction and supervision of a medical professional.