<qa>  What is the most common side effect of the drug?Patients will be assessed in the clinic every two months after starting treatment. Hydroxyurea will be started at 10 mgkgday, and increased by 5 mgkgday at eight week intervals if needed to reach a maximum tolerated dose MTD. The most common side effect of the drug is a drop in infection fighting cells, or white blood cells, so the medication will be started at a low dose and the dose will be increased only if it is safe to do so. 