Pronunciation: ko-SEL-u-go
Generic name: selumetinib
Dosage form: oral capsule (10 mg, 25 mg)
Drug class: Multikinase inhibitors

Medically reviewed by Carmen Pope, BPharm. Last updated on Aug 28, 2025.

What is Koselugo?

Koselugo (selumetinib) is a targeted oral MEK inhibitor used to treat neurofibromatosis type 1 (NF1) in children aged 2 years and older who have symptomatic, inoperable plexiform neurofibromas (PN). It is taken as an oral capsule twice a day.

Koselugo gained FDA approval on April 10, 2020. There is no generic version available.

How Does Koselugo Work?

MEK Inhibitor Mechanism

Koselugo works by targeting a specific cellular signaling pathway that is overactive in children with NF1. A genetic mutation leads to overactivity of the RAF-MEK-ERK signaling pathway, which normally controls cell growth and division. When it is stuck in the "on" position, it causes uncontrollable cell growth, forming the benign tumors called neurofibromas that are characteristic of NF1.

Koselugo specifically blocks MEK1 and MEK2 proteins, key switches in the middle of the overactive pathway. This breaks this chain of communication, blocking the signal for excessive cell growth.

Koselugo (selumetinib) is a kinase inhibitor.

Koselugo Side Effects

Common Side Effects

The most common side effects of Koselugo are:

• Diarrhea
• Vomiting
• Stomach-area pain
• Nausea
• Dry skin
• Muscle and bone pain
• Feeling of tiredness or lacking energy
• Fever
• Sores in your mouth
• Headache
• Redness around the fingernails
• Itching.

Serious Koselugo Side Effects and Warnings

Koselugo may cause the following serious side effects:

Heart problems. Koselugo can lower the amount of blood pumped by your heart, which is common and can also be severe. Your healthcare provider will do tests before and during treatment with this medicine to check how well your heart is working.

Tell your healthcare provider right away if you get any of the following signs or symptoms:

• persistent coughing or wheezing
• shortness of breath
• swelling of your ankles and feet
• tiredness
• increased heart rate.

Eye problems and vision changes. Koselugo can cause eye problems that can lead to blindness. Your healthcare provider will check your vision before and during treatment with this medicine.

Report these vision symptoms immediately:

• blurred vision
• loss of vision
• dark spots in your vision (floaters)
• other changes to your vision.

Severe diarrhea. Diarrhea is common with Koselugo and can also be severe. Tell your healthcare provider right away the first time that you get diarrhea during treatment with this medicine. Your healthcare provider may give you medicine to help control your diarrhea and may tell you to drink more fluids.

Skin rashes. Skin rashes are common with Koselugo and can also be severe. Tell your healthcare provider if you get any of the following signs or symptoms:

• a rash that covers a large area of your body
• peeling skin
• blisters.

Muscle problems (rhabdomyolysis). Muscle problems are common with Koselugo and can also be severe. Treatment may increase levels of an enzyme in your blood called creatine phosphokinase (CPK), which may be a sign of muscle damage. Your healthcare provider should do a blood test to check your blood levels of CPK before you start taking Koselugo and during treatment. Tell your healthcare provider right away if you get any of the following signs or symptoms:

• • muscle aches or pain
  • muscle spasms and weakness
  • dark, reddish urine.

Your healthcare provider may change your dose, temporarily stop, or permanently ask you to stop taking Koselugo if you have any of these side effects.

Important: It is not known if Koselugo is safe and effective in children under 2 years of age.

These are not all of the possible side effects of Koselugo. Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.

Related/similar drugs

Who Should Not Take Koselugo: Contraindications

There are no contraindications to Koselugo.

Before Taking

Before taking Koselugo, tell your healthcare provider about all of your child's medical conditions, including if they:

• have heart problems
• have eye problems
• have liver problems
• are pregnant or plan to become pregnant
• are breastfeeding or plan to breastfeed.

Age restriction: Koselugo should not be given to a child younger than 2 years old.

Koselugo During Pregnancy and Breastfeeding

Pregnancy warnings: Koselugo can harm an unborn baby. Your healthcare provider should check to see if older children could be pregnant before they begin treatment with Koselugo.

Birth control requirements:

• Females who can become pregnant should use effective birth control (contraception) during treatment with Koselugo and for 1 week after their last dose.
• Males with female partners who can become pregnant should use effective birth control (contraception) during treatment with Koselugo and for 1 week after their last dose.

Tell your healthcare provider right away if you become pregnant or think you may be pregnant during treatment with this medication.

Breastfeeding considerations:

• It is not known if Koselugo passes into breast milk.
• Do not breastfeed during treatment with Koselugo and for 1 week after your last dose.
• Talk to your healthcare provider about the best way to feed your baby during this time.

How do I Take Koselugo?

Take Koselugo exactly as prescribed by your doctor. Follow all directions on your prescription label and read all medication guides or instruction sheets.

Key dosing guidelines:

• Your healthcare provider decides the correct dose for you based on body surface area (height and weight).
• Your dose needs may change if you gain or lose weight, or if you are still growing.
• Do not change your dose or dosing schedule without your doctor's advice.

Daily Koselugo Dosing Schedule

• Koselugo is taken 2 times a day.
• Can be taken with or without food.
• Take each dose at the same time each day, about 12 hours apart.
• Take with a full glass of water.
• Swallow the capsule whole and do not crush, chew, break, or open it.
• Tell your doctor if your child has trouble swallowing the capsule whole.

What to do After Taking Koselugo

If you vomit shortly after taking Koselugo:

• Do not take another dose.
• Wait until your next scheduled dose time to take the medicine again.

Managing diarrhea:

• Tell your doctor if you start having loose stools (diarrhea).
• You may be given anti-diarrhea medicine.
• Drink plenty of liquids, including electrolyte replacement solutions, if you have diarrhea.
• Keep using this medicine for as long as your doctor has prescribed.

Required Monitoring During Treatment

Heart monitoring: Koselugo can cause your heart to pump less blood than normal. Your heart function will need to be checked every 3 to 6 months while you are taking this medicine.

Vision monitoring: Koselugo can also cause eye problems that may lead to blindness. You will need frequent vision exams, and your next dose may be delayed based on the results.

Koselugo Dosing Information for NF1 Treatment

Standard Pediatric Koselugo Dosing

Usual Koselugo dose for children 2 years and older with NF1:

• 25 mg/m² orally 2 times a day (approximately every 12 hours).
• Continue until disease progression or unacceptable toxicity occurs.

Liver Impairment Dosing Adjustments

Moderate liver impairment (Child-Pugh B): Reduce the recommended dosage to 20 mg/m² orally 2 times a day for patients.

Severe liver impairment (Child-Pugh C): The dosage has not been established. 

Missed Dose and Overdose Information

What to do if You Miss a Koselugo Dose

Take the medicine as soon as you can, but skip the missed dose if your next dose is due in less than 6 hours. Do not take two doses at one time.

Koselugo Overdose Emergency

What happens if I overdose? Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

What to Avoid While Taking Koselugo

Dietary restrictions:

• Do not drink grapefruit juice, eat grapefruit, or take supplements that contain grapefruit during treatment with Koselugo.
• Avoid St. John's wort during treatment.

Koselugo Drug Interactions

Tell your healthcare provider about all the medicines you take, including prescription and over-the-counter medicines, vitamins, or herbal supplements. Especially tell your healthcare provider if you are taking aspirin, blood thinners, or other medicines to treat blood clots. Koselugo contains vitamin E, which may increase your risk of bleeding.

Special bleeding risk warning: Tell your healthcare provider if you take aspirin, blood thinners, or other medicines to treat blood clots. Koselugo contains vitamin E, which may increase your risk of bleeding.

Storage

How to Store Koselugo:

• Store at room temperature away from moisture and heat.
• The bottle of Koselugo contains a desiccant packet to reduce moisture. Do not throw away the desiccant packet.
• Keep it in its original bottle. Keep the bottle tightly closed.
• Keep out of the reach of children.

Koselugo Ingredients and Manufacturer Information

Active and Inactive Ingredients

Active ingredient: selumetinib.

Inactive ingredients: Capsule contains: vitamin E, polyethylene glycol succinate.

10 mg capsule shell contains: hypromellose, carrageenan, potassium chloride, titanium dioxide, carnauba wax, and purified water. The 10 mg capsule printing ink contains: shellac, iron oxide black, propylene glycol, and ammonium hydroxide.

25 mg capsule shell contains: hypromellose, carrageenan, potassium chloride, titanium dioxide, FD&C blue 2, ferric oxide yellow, purified water, carnauba wax, and/or corn starch. The 25 mg printing ink contains: ferric oxide red, ferric oxide yellow, FD&C Blue 2 aluminum lake, carnauba wax, shellac, and glyceryl monooleate. 

Manufacturer Information

Koselugo (selumetinib) is made by AstraZeneca Pharmaceuticals LP, located in Wilmington, DE. 

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Medical Disclaimer