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A 16-year-old boy comes to the emergency department due to a day of severe left thigh pain. He was diagnosed with sickle cell disease during childhood. The boy has had several episodes of unbearable pain in his abdomen, thighs, and lower back that have required 6 separate hospitalizations for pain management and intravenous fluids. Temperature is 36.7 C (98 F), blood pressure is 120/80 mm Hg, pulse is 100/min, and respirations are 16/min. Pulse oximetry shows an oxygen saturation of 97%. Examination shows an uncomfortable-appearing boy. Lungs are clear to auscultation. The left thigh is tender to palpation but has no erythema or swelling. The patient is able to bear weight and has no pain elsewhere. Adequate hydration and analgesics are administered, with improvement of symptoms. On discharge from the hospital, the patient's hematologist counsels him about starting a first-line medication to decrease the frequency of his pain crises. What is the most common potential side effect of this first-line medication?