This medication may cause very serious blood disorders (decreased bone marrow function leading to low number of blood cells such as white cells, red cells, and platelets). This effect can weaken the immune system/increase the risk of infection, cause anemia, or cause your body to bruise or bleed easier. Tell your doctor right away if you develop any of the following: signs of infection (such as sore throat that doesn't go away, fever), easy bruising/bleeding, unusual tiredness, fast/pounding heartbeat.

When used in high doses, cladribine may cause very serious kidney or nerve problems. Nerve problems may also rarely occur in patients taking the normal dose of cladribine. Get medical help right away if you develop any of the following: change in the amount of urine, weakness/numbness/tingling in your hands/feet, or inability to move your arms/legs.

Cladribine may increase your risk of getting a rare but very serious (possibly fatal) brain infection (progressive multifocal leukoencephalopathy-PML). Get medical help right away if you have any of these side effects: clumsiness, loss of coordination/balance, weakness, sudden change in your thinking (such as confusion, difficulty concentrating, memory loss), difficulty talking, seizures, or sudden vision changes. See also Side Effects section.