Cladribine is used for the treatment of hairy cell leukemia and multiple sclerosis (MS). As a purine analog, it is a synthetic anti-cancer agent that also suppresses the immune system. Chemically, it mimics the nucleoside adenosine and thus inhibits the enzyme adenosine deaminase, which interferes with the cell's ability to process DNA. It can be distinguished from other chemotherapeutic agents affecting purine metabolism in that it is cytotoxic to both actively dividing and quiescent lymphocytes and monocytes, inhibiting both DNA synthesis and repair. Cladribine injection is a potent antineoplastic agent with potentially significant toxic side effects. In MS, the novel mechanism of action of cladribine is expected to reduce inflammation, autoimmune effects and autoreactive cell damage, thereby improving the integrity of the blood–brain barrier. Thus, the effects of cladribine may target some of the key events that are central to the pathophysiology of MS.