Besremi and Essential Thrombocythemia: What You Need to Know

Written by Michelle Vermeulen, PharmD
Medically Reviewed by Beth Johnston, BCPS, PharmD on September 09, 2026

Essential thrombocythemia is a rare blood disorder in which your bone marrow makes too many platelets, the blood cells responsible for clotting. It’s one of a group of conditions called myeloproliferative neoplasms. Having too many platelets raises your risk of blood clots and, in some cases, bleeding. Over time, essential thrombocythemia can also raise your risk of the bone marrow disease myelofibrosis or a type of blood cancer called acute myeloid leukemia. Many people with essential thrombocythemia don’t have symptoms at first, which is why regular blood tests matter.

Besremi (ropeginterferon alfa-2b-njft) is a lab-made version of a protein your body naturally produces called interferon alfa. It’s given as an injection under the skin. In August 2026, the FDA expanded Besremi’s approval to include essential thrombocythemia. It was already approved for a related condition, polycythemia vera.

This article covers how Besremi works, who it’s approved for, how it’s given, what clinical studies found, and its side effects and drug interactions.

In essential thrombocythemia, certain genetic changes (mutations) in your bone marrow cells cause them to make platelets faster than normal. Besremi attaches to a receptor on cells in your bone marrow called the interferon alfa receptor. This sets off a chain of signals inside the cell that changes which genes get turned on and off. The exact way this slows down extra platelet production isn’t fully understood, but it’s thought to work against the overactive marrow cells regardless of which specific mutation is driving your condition.

Besremi is approved for the treatment of adults with essential thrombocythemia. The clinical study that supported this approval enrolled adults whose essential thrombocythemia hadn’t responded well to (or who couldn’t tolerate) hydroxyurea, a commonly used first treatment.

Your health care provider will consider things such as how long you’ve had essential thrombocythemia, whether you’ve tried other treatments, your platelet and white blood cell counts, and your overall health before recommending Besremi. Besremi shouldn’t be used if you have certain conditions, including a history of severe depression or suicidal thoughts, moderate to severe liver problems, an untreated autoimmune disease, or if you’ve had an organ or stem cell transplant and take medicines that suppress your immune system. Tell your health care provider about your full medical history, including mental health, thyroid, heart, liver, and kidney conditions, before starting treatment.

If you can become pregnant, your health care provider will do a pregnancy test before you start Besremi, as it can harm a developing fetus. You’ll need to use effective birth control during treatment and for at least eight weeks after your last dose.

Besremi comes in a single-dose prefilled syringe or a prefilled pen injector, and you inject it under the skin of your stomach area or the top of your thighs. For essential thrombocythemia, treatment typically starts at a dose of 250 micrograms (mcg). After two weeks, your dose will likely increase to 350 mcg and after four weeks, to a maintenance dose of 500 mcg. You’ll then keep injecting Besremi every two weeks at that dose unless your health care provider needs to adjust it for side effects.

A caregiver can give you the injection, or you can learn to do it yourself with training from your health care provider. Rotate the injection site each time, and don’t inject into skin that’s irritated, bruised, or scarred. Your health care provider will check your complete blood count (a test that measures blood cells, including platelets) every two weeks while your dose is being adjusted, then every three to six months once you’re on a steady dose.

Don’t skip doses or stop Besremi on your own, even if you’re feeling well. If you think you’ve missed a dose or aren’t sure what to do, call your health care provider rather than guessing.

Besremi’s approval for essential thrombocythemia is based on a study called SURPASS ET. This was a randomized, open-label study, meaning participants were assigned by chance to one of two treatments, and both they and their health care providers knew which medicine they were getting. SURPASS ET compared Besremi to anagrelide, another medicine used for essential thrombocythemia, in 174 adults whose essential thrombocythemia hadn’t responded well to or who couldn’t tolerate hydroxyurea. Ninety-one people received Besremi and 83 received anagrelide. 

At the start of the study, participants were, on average, in their early 60s (aged 20-83 years), and just over half were female. Ninety-six percent of participants were Asian, and 4% were White. Everyone in the study had already tried hydroxyurea. People treated with Besremi injected it every two weeks, starting at 250 mcg and increasing to 500 mcg over four weeks, while the anagrelide group took that medicine by mouth. The main comparison period ran for 12 months.

The study measured a combined outcome researchers called a “durable modified response,” which required a participant to have their blood counts return to a safer range, have no worsening of an enlarged spleen, and have no bleeding or clotting events, all sustained at both 9 and 12 months of treatment.

About 37% of people who used Besremi reached this durable response at months 9 and 12, compared with about 4% of people who took anagrelide, a difference of about 34 percentage points. People who used Besremi were also less likely to have a blood clot or serious bleeding event over the 12-month study period than people who took anagrelide. Your results may differ from what was seen in clinical studies.

Like all medicines, Besremi can cause side effects. This section doesn’t include every possible side effect. Tell your health care provider about any symptom that concerns you or doesn’t improve, even if it’s not listed here.

Common side effects

  • Increased liver enzymes on blood tests
  • Tiredness (fatigue)
  • Low red blood cell count (anemia)
  • Lower white blood cell or neutrophil counts
  • Itching (pruritus)
  • Fever
  • Hair thinning or alopecia
  • Headache
  • Diarrhea and nausea
  • Flu-like symptoms and muscle aches

Serious side effects

  • New or worsening depression, thoughts of suicide, or other mood and behavior changes
  • Thyroid problems or high blood sugar
  • Heart problems or blood clots
  • Severe low blood cell counts, infections, or bleeding
  • Serious allergic reactions
  • Inflammation of the pancreas (pancreatitis) or intestines (colitis)
  • Lung problems
  • Eye problems, including changes to the retina
  • Liver or kidney problems

If you have symptoms that feel serious or life-threatening, seek medical care right away.

How to manage Besremi side effects

Mood and behavior changes. Besremi can cause or worsen serious mental health problems, including depression and suicidal thoughts, even in people with no history of them. In studies, about 10% of people using Besremi for essential thrombocythemia had a psychiatric side effect, most often depression or a low mood.

  • Tell your health care provider right away or have a caregiver or family member tell them about any new or worsening depression, anxiety, trouble sleeping, aggression, or thoughts of self-harm.
  • If you have thoughts of hurting yourself, stop using Besremi and get medical help right away.
  • Your health care provider may recommend a mental health evaluation or adjust your dose. In many cases, these symptoms improve after stopping Besremi.

Low blood cell counts, infections, and bleeding. Besremi can lower your platelet, red blood cell, and white blood cell counts, and low platelets increase your bleeding risk. In studies, some people developed anemia, low white blood cell counts, or bleeding events such as gum bleeding or nosebleeds. Infections occurred in almost half of people treated with Besremi for essential thrombocythemia, though most weren’t serious. 

  • Watch for unusual tiredness, easy bruising, frequent nosebleeds, bleeding gums, fever, or chills, and call your health care provider if these occur.
  • Keep every scheduled blood count check, as your health care provider uses these to catch changes early and adjust your dose if needed.
  • Don’t adjust your Besremi dose on your own if your blood counts change; your health care provider will decide whether to reduce your dose, pause treatment, or make another change.

Liver-related changes. Increases in liver enzymes on blood tests are the most common side effect reported with Besremi, and they’re usually mild. Less often, Besremi can cause more serious liver problems, and it shouldn’t be used at all in people with moderate or severe liver impairment.

  • Your health care provider will check your liver enzymes before you start Besremi and regularly during treatment.
  • Tell your health care provider if you notice yellowing of your skin or eyes, dark urine, swelling in your stomach area, unusual tiredness, or loss of appetite.
  • Depending on how high your liver enzymes rise, your health care provider may lower your dose or, in some cases, stop Besremi.

Eye problems. Besremi can affect the retina, the light-sensing tissue at the back of the eye, in ways that can lead to blurred vision or, rarely, vision loss. About 23% of people using Besremi for essential thrombocythemia had some type of eye-related side effect in studies, most commonly blurred vision or dry eyes.

  • Get an eye exam before starting Besremi, especially if you have diabetes or high blood pressure, and have follow-up eye exams during treatment.
  • Tell your health care provider right away about blurred vision, seeing floaters or flashes of light, eye pain or redness, or any sudden vision changes.
  • Your health care provider may stop Besremi if you develop a new or worsening eye problem.

Thyroid and blood sugar changes. Interferon medicines can trigger or worsen autoimmune conditions, including thyroid problems and, less commonly, new-onset diabetes.

  • Watch for symptoms such as unusual tiredness, weight changes, feeling unusually cold or hot, or increased thirst and urination.
  • Your health care provider may check your thyroid health or blood sugar levels periodically during treatment.
  • If a thyroid or blood sugar problem develops and can’t be managed while you continue treatment, your health care provider may need to stop Besremi.

Heart problems and blood clots. Interferon medicines, including Besremi, have been linked to heart problems such as abnormal heart rhythms and reduced blood flow to the heart, as well as blood clots. Besremi shouldn’t be used if you have uncontrolled high blood pressure, heart failure, or a serious heart rhythm problem or had a recent heart attack or stroke.

  • Tell your health care provider about any personal or family history of heart problems or blood clots before starting treatment.
  • Get medical help right away for chest pain, trouble breathing, a fast or irregular heartbeat, or sudden swelling, numbness, or weakness, especially on one side of the body.
  • If you have a history of heart or clotting problems, your health care provider will likely monitor you more closely during treatment.

Tell your health care provider about any side effect, including ones not listed here, especially if it’s new, worsening, or worrying you.

A drug interaction happens when another medicine, supplement, or substance changes how well a medicine works or how it affects your body. This isn’t a full list of Besremi’s interactions, so always give your health care provider and pharmacist a complete list of everything you take.

Medicines processed by liver enzymes with a narrow safety margin. Besremi can slow down certain liver enzymes (part of the cytochrome P450 system) that break down other medicines. For a few medicines where the difference between an effective dose and a harmful one is small, this can raise the amount of that medicine in your body and increase the risk of side effects. Talk to your health care provider about all medicines you take. They can help you determine if any of your medicines need to be adjusted or monitored more closely while taking Besremi.

Medicines that also lower blood cell counts. Combining Besremi with other medicines that suppress bone marrow function can add to its effect on your blood counts, raising the risk of low platelets, low red blood cells, or low white blood cells. Examples include hydroxyurea and certain chemotherapy medicines. This combination is generally avoided. If your health care provider decides you need both, they’ll likely check your blood counts more often to watch for excessive drops.

Narcotics, sleep medicines, and sedatives. Besremi can add to the drowsiness, dizziness, and other nervous-system effects of opioid pain medicines, sleep aids, and sedatives. Examples include benzodiazepines such as lorazepam, oxycodone, and zolpidem. Combining these with Besremi is generally avoided. If you need one of these medicines while using Besremi, your health care provider will watch you closely for excessive sleepiness, confusion, or slowed breathing and won’t have you adjust either medicine’s dose on your own.

Common questions about Besremi interactions

Does Besremi interact with alcohol?

Because Besremi can affect your liver and your mood, ask your health care provider whether it’s safe for you to drink alcohol during treatment, and how much, if any, is appropriate for you.

What should I avoid while using Besremi?

Avoid driving or using heavy machinery if Besremi makes you feel dizzy or drowsy or if you have hallucinations. Also avoid starting or stopping any other medicine, including over-the-counter products and supplements, without checking with your health care provider first.

  • Tell your pharmacist and other health care providers about every medicine, supplement, and vitamin you take, not just prescription medicines.
  • Ask before starting or stopping any medicine while you’re using Besremi.
  • Keep all your scheduled blood tests and other monitoring appointments.
  • Report new or worsening symptoms to your health care provider, even if you’re not sure they’re related to Besremi.

Your health care provider can adjust your treatment plan as your body responds. Don’t change your dose or stop Besremi on your own; work with your health care provider on any changes.

The drugmaker offers a patient support program called SOURCE that may help lower your monthly copay to as little as $0. For questions or more information, you can call 800-700-5053.

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