Stopping hydroxyurea allows the condition it was managing to reassert itself, often within weeks to months. For people with sickle cell disease, that means a return of painful crises, a higher chance of acute chest syndrome, and loss of the protective fetal hemoglobin the drug was boosting. For those taking it for blood cancers like essential thrombocythemia or polycythemia vera, platelet or red blood cell counts climb back up, and the risk of dangerous clots rises again. The timeline and severity of these rebounds vary depending on the condition, how long you were on the drug, and whether you taper off or quit abruptly. But some effects of stopping are actually desirable, which is why doctors sometimes plan a discontinuation on purpose.
Return of Sickle Cell Crises
Hydroxyurea is one of the most studied drugs in sickle cell disease, and the landmark trial that established its benefit showed just how much it does. Patients on hydroxyurea had roughly half the rate of painful crises compared to those on placebo, with a median of about 2.5 crises per year versus 4.5. They also went longer before their first crisis and were far less likely to develop acute chest syndrome, a potentially fatal lung complication.1PubMed. Effect of Hydroxyurea on the Frequency of Painful Crises in Sickle Cell Anemia When you stop the drug, those protections fade. The mechanism is straightforward: hydroxyurea raises levels of fetal hemoglobin, a form of hemoglobin that interferes with the sickling process. Once you stop taking the drug, fetal hemoglobin production gradually declines, and red blood cells start sickling at higher rates again.
The decline does not happen overnight. Fetal hemoglobin levels drop over weeks, and most people notice an increase in symptoms within one to three months of stopping. A long-term follow-up study spanning more than 17 years found that pulmonary complications were the leading cause of death in sickle cell patients, and the vast majority of those deaths occurred in patients who had either never taken hydroxyurea or had used it for fewer than five years.2PubMed Central. The Risks and Benefits of Long-term Use of Hydroxyurea in Sickle Cell Anemia: A 17.5 Year Follow-Up That does not prove stopping the drug caused those deaths, but it paints a clear picture: sustained use is associated with better survival, and shorter use or none at all tracks with worse outcomes.
What Happens in Blood Cancers
Hydroxyurea is also a mainstay treatment for myeloproliferative neoplasms, conditions where the bone marrow overproduces blood cells. In essential thrombocythemia, the main danger is excessive platelets leading to clots. A trial comparing hydroxyurea to no cytoreductive treatment found that only about 4 percent of patients on the drug had a clot, versus 24 percent of controls.3PubMed. Hydroxyurea for patients with essential thrombocythemia and a high risk of thrombosis Stopping hydroxyurea in these patients allows platelet counts to rebound, sometimes within a couple of weeks, restoring the elevated thrombotic risk that the drug was holding in check.
In polycythemia vera, where the problem is too many red blood cells, discontinuation similarly leads to rising hematocrit and renewed risk of stroke, heart attack, or other clotting events. The rebound is not theoretical; it is the expected physiological response once the drug’s suppressive effect on the bone marrow wears off. For patients in this group, roughly 20 to 25 percent eventually develop resistance or intolerance to hydroxyurea and have to switch to alternatives like anagrelide or interferon-alpha.4PubMed Central. Therapeutic options for patients with polycythemia vera and essential thrombocythemia refractory/resistant to hydroxyurea 5PubMed. Therapeutic options for essential thrombocythemia and polycythemia vera When stopping is necessary, the switch typically happens under close monitoring so that blood counts don’t spike dangerously before the new therapy takes effect.
Side Effects That Reverse After Stopping
Not everything about discontinuation is bad news. Several of hydroxyurea’s unwanted effects are reversible once the drug clears the system, and in some cases stopping is the whole point.
Fertility is the most studied example. Hydroxyurea suppresses sperm production, sometimes severely. Animal research has shown that after four months off the drug, sperm density and motility improve substantially, and testosterone levels return to normal ranges. Males that had been on the drug were able to sire viable offspring after recovery, though at a somewhat lower rate than males that were never treated.6PubMed Central. Resumption of Spermatogenesis and Fertility Post Withdrawal of Hydroxyurea Treatment In human case reports, the picture is encouraging: men with polycythemia vera or essential thrombocythemia who stopped hydroxyurea showed significant improvements in semen quality, and spontaneous pregnancies followed.7PubMed Central. Impact of Hydroxyurea to Treat Haematological Disorders on Male Fertility: Two Case Reports and a Systematic Review The recovery was somewhat less clear-cut in men with sickle cell disease, possibly because the disease itself can impair fertility, but the overall trend pointed toward improvement after discontinuation.
Other reversible side effects include certain skin changes (darkened nails, skin hyperpigmentation, and leg ulcers that can develop with long-term use) and gastrointestinal problems. In one reported case, a woman who developed ileocecal ulcers after 10 years of hydroxyurea therapy for thrombocythemia saw significant healing within six months of stopping the drug.8PubMed Central. Hydroxyurea-related ileocecal region ulcers as a rare complication: A case report Blood counts, too, normalize relatively quickly. The bone marrow suppression that lowers white blood cell and platelet counts while on the drug reverses within weeks of stopping, which is why routine blood monitoring is standard practice during treatment.
Stopping Before Pregnancy
Planned discontinuation is most common around pregnancy. Hydroxyurea is classified as potentially harmful to a developing fetus based on animal studies and limited human data, and current guidance recommends stopping it at least three months before attempting to conceive.9Journal of Obstetrics and Gynaecology Canada. Pregnancy and Neonatal Outcomes Following Hydroxyurea Exposure: A Systematic Review The three-month window gives the body time to clear the drug and allows a full cycle of new sperm or egg development without exposure.
The data here are nuanced. A study of pregnancy outcomes in women with sickle cell disease found that using hydroxyurea up to the time of conception but not during pregnancy did not significantly increase the risk of miscarriage or stillbirth. However, women who continued the drug through both conception and pregnancy had roughly double the odds of miscarriage or stillbirth, and full-term infants were nearly three times as likely to have low birth weight.10PubMed Central. Pregnancy Outcomes with Hydroxyurea Use in Women with Sickle Cell Disease Researchers have concluded that the drug appears safe up to the time of conception but that caution is warranted during pregnancy itself.11Blood. Hydroxyurea Use and Outcomes of Pregnancy in Sickle Cell Disease
For women with sickle cell disease, stopping hydroxyurea during pregnancy creates a real tension. The drug was controlling their disease, and pregnancy itself increases the risk of sickle cell complications like pain crises and acute chest syndrome. Doctors typically manage this gap with closer monitoring, transfusions if needed, and sometimes other supportive treatments. It is one of those clinical situations where the risks of continuing and the risks of stopping both need to be weighed carefully for each person.
The Leukemia Question
Some people stop hydroxyurea, or resist starting it, because of concerns about cancer. Hydroxyurea works by interfering with DNA synthesis, and anything that does that raises the theoretical concern of causing mutations that could lead to leukemia. The worry is not entirely hypothetical: in patients with polycythemia vera and essential thrombocythemia, studies have reported that roughly 5 to 10 percent of those on long-term hydroxyurea developed acute leukemia. But whether hydroxyurea actually caused those cases or whether the underlying myeloproliferative disease was the real driver remains genuinely uncertain.
For sickle cell disease, the evidence is more reassuring. A nine-year follow-up of the original sickle cell trial found very little risk of leukemia, and the researchers concluded that the risk of dying from sickle cell complications was at least ten times greater than the estimated risk of developing leukemia from the drug.12JAMA. Effect of Hydroxyurea on Mortality and Morbidity in Adult Sickle Cell Anemia: Risks and Benefits Up to 9 Years of Treatment The long-term follow-up data similarly showed that rates of malignancy were similar whether patients took hydroxyurea or not.2PubMed Central. The Risks and Benefits of Long-term Use of Hydroxyurea in Sickle Cell Anemia: A 17.5 Year Follow-Up So while the theoretical risk exists and is worth monitoring, stopping the drug specifically to avoid leukemia in sickle cell disease trades a clear, measurable benefit for a small and uncertain risk. The math overwhelmingly favors staying on it.
Why People Actually Stop
Despite the evidence that hydroxyurea works, discontinuation rates are high. In a large claims-based study, nearly 59 percent of sickle cell patients prescribed hydroxyurea stopped taking it within 12 months. Average adherence was only about 52 percent of the days they were supposed to be on it, and just over one in five patients hit the 80 percent threshold that is generally considered adequate adherence.13PubMed Central. Treatment patterns and economic burden of sickle-cell disease patients prescribed hydroxyurea: a retrospective claims-based study
Qualitative research has explored why. About 70 percent of nonadherence falls into the “unintentional” category: forgetting doses, running out of medication, or life circumstances that disrupted the routine. The remaining 30 percent is intentional, driven by negative perceptions of the drug or a general aversion to taking medication.14PubMed Central. Intentional and unintentional nonadherence to hydroxyurea among people with sickle cell disease: a qualitative study Worry about side effects was common among both people who were currently taking hydroxyurea and those who had stopped, while people not on the drug were more likely to say their doctor never recommended it in the first place, or that they had tried it and felt it didn’t work.15Frontiers in Genetics. Barriers to hydroxyurea use from the perspectives of providers, individuals with sickle cell disease, and families: Report from a U.S. regional collaborative
Particularly striking is a subset of patients who stopped because they perceived their pain actually increased on the drug, or because they felt it had stopped being effective. Some made that decision without consulting their doctor.16PLOS ONE. From trust to skepticism: An in-depth analysis across age groups of adults with sickle cell disease on their perspectives regarding hydroxyurea This highlights a real challenge: hydroxyurea takes time to reach its full effect, and pain crises can still occur even when the drug is working. A crisis that happens during treatment can feel like evidence of failure, even if the drug has reduced overall crisis frequency. The gap between how a treatment works statistically and how it feels day-to-day is one of the hardest things in medicine to communicate.
The Cost of Gaps in Treatment
Intermittent use and discontinuation are not just clinical problems; they carry an economic dimension. In the same claims study that tracked adherence, patients averaged over 13 days in the hospital per sickle-cell-related admission, and nearly two-thirds had at least one hospitalization during the year. Average annual sickle-cell-related costs came to roughly $28,000 per patient, with inpatient stays accounting for most of that.13PubMed Central. Treatment patterns and economic burden of sickle-cell disease patients prescribed hydroxyurea: a retrospective claims-based study These figures represent a population with low average adherence, and the implication is clear: underuse of a relatively inexpensive daily pill leads to expensive emergency care. Hydroxyurea itself costs far less than a single hospital stay for an acute crisis.
Restarting After a Break
If you have been off hydroxyurea for a while and want to restart, the process is generally straightforward but not instant. Doctors typically resume at a low dose and titrate upward based on blood counts, much as they did when first prescribing the drug. For sickle cell disease, it takes several weeks for fetal hemoglobin to start climbing again, and the full benefit may not return for two to three months. During this ramp-up period, you are relatively unprotected compared to someone on a stable dose.
For myeloproliferative neoplasms, blood counts tend to respond a bit faster, often within one to two weeks of restarting, but the optimal dose may need to be re-established through monitoring. This is why hematologists prefer that patients communicate before stopping rather than just running out of medication and restarting later on their own. An unplanned gap means an unplanned period of risk.
Children and Stroke Prevention
In children with sickle cell disease, hydroxyurea plays an especially important role in stroke prevention. A Belgian study tracking 72 pediatric patients found that among 34 children identified as at risk for stroke through screening, only one had a cerebrovascular event during nearly 100 patient-years of follow-up on hydroxyurea.17Blood. Hydroxyurea for sickle cell disease in children and for prevention of cerebrovascular events: the Belgian experience Stopping the drug in a child at high stroke risk is a serious decision because the window for a first stroke in sickle cell disease is during childhood, and the consequences can be devastating and permanent.
Parents sometimes worry about keeping a child on a chemotherapy-derived drug for years or even indefinitely. That concern is understandable, and it overlaps with the same anxieties adults have about long-term side effects. But the long-term safety data extending well past a decade suggest that continuous use in children carries far less risk than the strokes, organ damage, and pain crises it prevents. Discussions about stopping should always happen with a hematologist, and decisions should weigh the child’s individual risk profile rather than general anxiety about the word “chemotherapy.”
When Stopping Is Medically Necessary
Sometimes discontinuation is not a choice but a medical requirement. The most common reasons doctors stop the drug include dangerous drops in blood counts (severe neutropenia or thrombocytopenia), kidney or liver problems that impair the body’s ability to handle the drug, or specific adverse reactions like the ileocecal ulcers described earlier. In myeloproliferative conditions, the development of intolerance or resistance affects a sizable minority of patients and necessitates a switch to second-line therapy.
When stopping is medically necessary, the transition plan matters. For sickle cell disease, alternatives include L-glutamine, crizanlizumab, and voxelotor, though each works by a different mechanism and none replicates exactly what hydroxyurea does. Chronic transfusion programs are another option, particularly for stroke prevention in children. For blood cancers, anagrelide and interferon-alpha remain the primary second-line agents. The key point is that stopping hydroxyurea should prompt a conversation about what comes next, because the underlying disease does not stop just because the drug does.