Patients may experience mild to severe infusion reactions such as chills, itching, high temperature, hives, difficulty breathing, and chest pain. If patients experience any symptoms during the infusion, it is very important to let the nurse know immediately. The most serious potential side effect of rituximab is progressive multifocal leukoencephalopathy (PML), an infection of the brain, which can manifest in a number of ways including trouble with gait, speech and memory.