The study is looking at how the medication *mitapivat* affects the need for blood transfusions in people with Sickle Cell Disease (SCD). **Mitapivat** is a drug that might help reduce the need for blood transfusions. A **placebo** is a pill with no medicine used to compare results. People aged 12 and older with SCD who have had at least one blood transfusion in the past year can join. They must also meet certain health requirements, like having specific levels of hemoglobin, a protein in red blood cells, and other signs of cell breakdown. Key Points: - The study involves taking either mitapivat or a placebo. - Participants must be available for study procedures and meet health criteria. - This study does not allow current use of certain SCD treatments other than *hydroxyurea*. If you have SCD and meet the criteria, this study might offer a chance to try a new treatment. Always talk to your doctor to understand the risks and benefits before participating.