Clomethiazole is a sedative. It's primary use is to treat alcohol withdrawal. However, it is often used in geriatric patients to reduce agitation and restlessness, to treat short term insomnia, and to relieve the symptoms of Parkinson's disease.
It's primary danger is that long-term use can lead to dependence and withdrawal can lead to the same consequences as alcohol or benzodiazepine withdrawal. In addition, an overdose will almost certainly be fatal, and it's effects are multiplied when combined with alcohol. As such, it is not recommended for use outside of a hospital setting, as other drugs, like diazepam, have the same effect but are much safer.