Hydroxyurea

• Sickle Cell Disease
• Essential Thrombocythemia
• Polycythemia Vera
• Chronic Myeloid Leukemia

Hydroxyurea, also called hydroxycarbamide and sold as Hydrea, Droxia, and Siklos, is one of the most clinically important oral disease-modifying drugs in modern hematology. Originally introduced as an anti-cancer agent, it has since become the cornerstone preventive therapy for sickle cell disease and a first-line cytoreductive option for several myeloproliferative neoplasms including polycythemia vera, essential thrombocythemia, and chronic-phase chronic myeloid leukemia in patients who cannot use tyrosine kinase inhibitors. Decades of follow-up data confirm that, when properly monitored, hydroxyurea reduces vaso-occlusive crises, hospitalizations, transfusion needs, and mortality in sickle cell disease, while in myeloproliferative disorders it lowers thrombotic events by controlling elevated counts. Despite its broad utility, hydroxyurea is a chemotherapy agent that requires careful dosing, regular blood monitoring, and disciplined contraception planning.

Mechanism of Action

Hydroxyurea inhibits ribonucleotide reductase, a tetrameric enzyme that converts ribonucleoside diphosphates into the deoxyribonucleotides required for DNA synthesis and repair. By depleting the deoxynucleotide pool, hydroxyurea arrests dividing cells in the S phase of the cell cycle, producing a relatively selective effect on rapidly proliferating populations such as bone marrow precursors and cancer cells. This is the basis for its myelosuppressive and anti-neoplastic activity, and it is why dose-limiting toxicity is reliably reflected in falling blood counts.

In sickle cell disease the picture is more complex. Hydroxyurea increases the production of fetal hemoglobin (HbF) in red cell precursors, probably through nitric-oxide-mediated activation of soluble guanylate cyclase and stress-erythropoiesis pathways. Because HbF does not participate in HbS polymerization, raising the percentage of HbF inside individual red cells reduces sickling under hypoxic stress. Beyond HbF induction, hydroxyurea lowers neutrophil and reticulocyte counts (both contribute to vaso-occlusion), reduces red cell adhesion to vascular endothelium, donates nitric oxide that promotes vasodilation, and reduces hemolysis. The combination shrinks the rate of painful vaso-occlusive crises by roughly half in adults and even more in well-monitored children. In polycythemia vera and essential thrombocythemia, the dominant mechanism is straightforward myelosuppression, controlling hematocrit or platelet count to reduce thrombosis risk.

Clinical Use

For sickle cell disease, the American Society of Hematology now recommends offering hydroxyurea to all patients aged 9 months and older regardless of clinical severity, given clear evidence that it reduces complications and may modify long-term organ damage. In adults, the standard threshold for starting therapy includes recurrent painful crises (three or more per year), prior acute chest syndrome, severe symptomatic anemia, or chronic disabling pain. Dose is escalated to maximum tolerated dose, typically defined by absolute neutrophil count between 2,000 and 4,000 cells/microliter without other toxicity, which produces the greatest HbF response and clinical benefit.

In polycythemia vera, hydroxyurea is offered when patients have high-risk features such as age over 60 or prior thrombosis, in addition to phlebotomy and low-dose aspirin. In essential thrombocythemia, similar high-risk indications drive its use. For chronic myeloid leukemia, hydroxyurea provides rapid cytoreduction for symptomatic leukocytosis but has been largely supplanted by tyrosine kinase inhibitors such as imatinib for long-term control. Comparative effectiveness data favor hydroxyurea over no cytoreduction in chronic leukemia and myeloproliferative settings, while in sickle cell disease, alternatives or adjuncts include voxelotor, crizanlizumab, L-glutamine, chronic transfusion programs, and curative hematopoietic stem cell transplantation. Patient selection requires reliable adherence, willingness to use contraception, ability to attend lab visits, and absence of pregnancy planning. Our hematology team coordinates monitoring and dose titration with primary care.

How to Take It

Hydroxyurea capsules and tablets are taken once daily, with or without food, at approximately the same time each day. Capsules should be swallowed whole; if a child or adult cannot swallow capsules, the Siklos tablet formulation has functional scoring for accurate fractional dosing, or the contents of a capsule may be carefully dispersed in a small volume of water and swallowed immediately, with attention to handling precautions. Pregnant women, women trying to conceive, and anyone wishing to avoid skin contact should not handle broken or opened capsules; if powder contacts skin, wash thoroughly with soap and water.

If a dose is missed, take it as soon as possible the same day. If it is the next day, skip the missed dose and resume the regular schedule; never double up. Adequate hydration is important to support kidney clearance; aim for at least 8 cups of water daily unless restricted for heart or kidney reasons. Store at room temperature in a tight container, away from children, since accidental ingestion can cause serious toxicity. The first weeks of therapy may produce mild nausea, headache, fatigue, and gradually deepening skin or nail pigmentation, which is cosmetic. Painful crisis frequency typically begins to decline after about three months, though full HbF response often requires six months or longer. Cholesterol-lowering and other chronic medications can usually be continued unchanged.

Monitoring and Follow-Up

Close laboratory monitoring is mandatory and is the most important safety practice for this drug. A complete blood count with differential and reticulocyte count is checked every two weeks during dose escalation, then every four to eight weeks at stable dose, with longer intervals only after prolonged stability. Target absolute neutrophil count is generally between 2,000 and 4,000 cells/microliter; platelets above 80,000; hemoglobin maintained at the patient's own steady-state value. Red flag numbers that prompt dose interruption include neutrophils below 2,000, platelets below 80,000, hemoglobin drop greater than 20 percent below baseline, or reticulocytes below 80,000 with hemoglobin below 9 g/dL.

In sickle cell disease, fetal hemoglobin percentage is checked at baseline and periodically; a rising HbF, often into the 15 to 30 percent range, correlates with clinical response. A comprehensive metabolic panel is checked every three to six months to follow renal and hepatic function. Pregnancy testing in women of reproductive potential is performed before initiation and as clinically indicated. Patients with sickle cell should also receive routine annual screenings for organ complications, including transcranial Doppler in children, retinal exams, and renal function. Communication of any new bruising, persistent fevers above 101 degrees Fahrenheit, severe fatigue, or non-healing leg ulcers is essential between visits. Additional patient resources are available from the American Society of Hematology.

Special Populations

Elderly patients tolerate hydroxyurea well overall but may have age-related declines in renal function that require lower starting doses and more frequent count monitoring. Renal impairment requires dose reduction; current guidance suggests initiating at 50 percent of standard dose when creatinine clearance is below 60 mL/min, with subsequent titration based on counts. Hepatic dysfunction is not a strict contraindication but warrants extra vigilance because liver enzymes can rise. Pediatric use is established in sickle cell disease starting at 9 months of age with weight-based dosing.

Pregnancy is a major concern: hydroxyurea is teratogenic in animals and is generally contraindicated during pregnancy. Effective contraception is required for women throughout treatment and for at least six months after the last dose, and for men, contraception is recommended for at least one year because hydroxyurea may cause oligospermia and possible sperm DNA damage. Sperm banking should be discussed with men of reproductive age before therapy. Breastfeeding is contraindicated because the drug is excreted into breast milk. Live vaccines such as MMR, varicella, and live influenza are generally avoided during therapy due to immunosuppression; inactivated vaccines including annual flu and pneumococcal vaccines are encouraged.

When to Contact Your Doctor

Call the office promptly for fever above 101 degrees Fahrenheit, persistent sore throat, mouth ulcers, unusual bruising or bleeding, painful or non-healing leg ulcers, blistering skin rash, severe abdominal pain, jaundice, dark urine, or a sudden drop in exercise tolerance. New shortness of breath, cough with sputum production, pleuritic chest pain, or hypoxia could indicate acute chest syndrome in sickle cell patients or pulmonary toxicity from the drug, and warrants emergency evaluation. Pregnancy, suspected pregnancy, or contraceptive failure should be reported immediately. Severe nausea or inability to keep fluids down, signs of dehydration, severe headache that is different from prior crises, focal weakness, or symptoms of stroke require urgent care. Do not skip scheduled blood draws even when feeling well; many problems are detected on labs before symptoms appear.

For management of sickle cell disease, polycythemia vera, or other hematologic conditions where hydroxyurea may be appropriate, contact us or schedule a visit. Detailed dosing, interaction tables, and frequently asked questions are provided on this page below.