Qualaquin (quinine sulfate), a malaria drug, should not be used off-label to prevent and treat nighttime leg cramps due to the risk for serious side effects, the FDA warned on Thursday.

Thirty-eight cases of serious side effects associated with quinine use — including two deaths — were reported to the FDA’s Adverse Event Reporting System between 2005 and 2008. Side effects included kidney damage, thrombocytopenia and other hematologic reactions, hearing loss, and cardiovascular problems. Most of these reports were the result of off-label use for leg cramps or restless leg syndrome, according to the agency.

As part of a new risk management plan, patients will now receive a medication guide when filling prescriptions for Qualaquin.