**1. Introduction**

Neurological disorders are devastating diseases which usually occur in the brain, spinal cord, cranial nerves, peripheral nerves, and so on. It has reported that there are more than 600 kinds of neuropathological conditions including epilepsy, brain tumor, Parkinson's disease, Alzheimer's disease, and stroke. Nowadays, it is estimated that more than 1 billion people suffer from neurological disorders, seriously affecting people's life quality [1]. These kinds of diseases are especially prevalent in developing countries at any stage of age [2, 3]. There are several factors contributing to etiology of neurological disorders such as aggravating tendency of aging population, irregular diet, and insufficient exercise [4].

Drug therapy is an important way for curing neurological diseases in the clinic. Nevertheless, serious neurological disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD) are usually incurable in late stages of diseases with current therapeutic intervention [5, 6]. In the meantime, drug treatment often becomes less effective and causes serious side effects due to individual differences. Taking

epilepsy as example, nearly 30% of epileptic patients are unable to obtain seizure control following treatment with marketed drugs [7, 8]. In addition, they have no significant effect on the improvement of cognitive dysfunction in patients with severe epilepsy [9]. Thus, it is essential for investigation of more effective and/or less toxic CNS targeted drugs.

Drug repurposing, also known as drug reprofiling or drug repositioning, includes the development of new uses and dosage forms for existing drugs or drug candidates. It is regarded as an economic and practical strategy [10]. Drug repurposing avoids the defects of new drug development. Compared to the drug repurposing, development of new drugs consumes much more time and huge investments. It is roughly reported that the cost from basic research for a new drug to clinical trials is 2.6 billion US dollars [11] and it often takes an average of 13–15 years [12]. Although more and more drug candidates are developed, many cases have failed in recent years [13]. Most of new drugs are withdrawn from the market due to unsatisfactory efficacy or intolerable side effects [14, 15]. Therefore, reusing existing drugs, namely, drug repurposing, has attracted great attention, as this approach has the capacity of saving cost and expediting drug development process.

The purpose of this chapter is to discuss the role of drug repurposing in human diseases especially neurological diseases and summarize repurposing candidates currently in clinical trials for neurological diseases and potential mechanisms as well as preliminary results. Subsequently we also list drug repurposing approaches and limitations and challenges in the future investigations.
