Which muscle relaxant is known for its unlabeled use in treating hiccups?

Baclofen is known for its unlabeled use in treating hiccups due to its ability to act as a central nervous system depressant that can inhibit reflexes associated with hiccups. This muscle relaxant primarily works by affecting the GABA-B receptors in the spinal cord, which leads to a decrease in excitatory neurotransmission. As a result, baclofen can effectively reduce involuntary muscle contractions, making it useful in situations where hiccups persist and other treatments fail.

Other muscle relaxants in the options have different primary uses and mechanisms that do not provide the same effect on the hiccup reflex. For instance, cyclobenzaprine is primarily used for muscle spasms related to acute musculoskeletal conditions, while diazepam mainly serves as an anxiolytic and has sedative properties rather than being primarily specified for hiccup treatment. Carisoprodol, like cyclobenzaprine, is mainly recommended for muscle relaxation and pain relief and does not have an established role in treating hiccups. Thus, baclofen stands out as the most relevant drug for this condition with its additional use beyond muscle relaxation.