Letermovir belongs to the class of medications called antivirals. It is used to prevent infection caused by cytomegalovirus (CMV) for adults who have had a stem cell transplant with cells from another person, who are at risk of developing CMV infection.

Letermovir works by preventing CMV from reproducing and spreading, causing infection.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

There are two forms of this medication: tablets and solution for intravenous (into a vein) infusion.

The recommended adult dose of letermovir is 480 mg taken by mouth, once daily. It should be started on or after the day you receive the stem cell transplant, but no later than 28 days after the transplant. It is generally taken until 100 days after the transplant.

The tablets should be taken at approximately the same time each day. Swallow the tablets whole with some fluid. Do not crush or chew the tablets. Letermovir may be taken with food or on an empty stomach.

If you are receiving the intravenous form of this medication, it will take approximately 1 hour for the infusion to complete.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor.

If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication in its original package at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Tablets

240 mg
Each yellow oval tablet, debossed with “591” on one side and the Merck logo on the other side contains 240 mg of letermovir. Nonmedicinal ingredients: colloidal silicon dioxide, croscarmellose sodium, magnesium stearate, microcrystalline cellulose and povidone 25; film-coating: hypromellose 2910, iron oxide yellow, lactose monohydrate, titanium dioxide, triacetin, and carnauba wax.

480 mg
Each pink, oval, bi-convex tablet, debossed with “595” on one side and the Merck logo on the other side, contains 480 mg of letermovir. Nonmedicinal ingredients: colloidal silicon dioxide, croscarmellose sodium, magnesium stearate, microcrystalline cellulose and povidone 25; film-coating: hypromellose 2910, iron oxide yellow, iron oxide red, lactose monohydrate, titanium dioxide, triacetin, and carnauba wax.

Solution for Injection

Each mL of clear, preservative-free sterile solution in single dose vials of either 240 mg or 480 mg per vial, contains 20 mg of letermovir. Nonmedicinal ingredients: hydroxypropyl betadex, sodium chloride, sodium hydroxide and water for injection. The amount of sodium hydroxide may be adjusted to achieve a pH of approximately 7.5.

Do not take this medication if you:

• are allergic to letermovir or any ingredients of the medication
• are taking the medication pimozide
• are taking any of the following medications:
• bosentan
• cyclosporine
• ergot alkaloids (e.g., dihydroergotamine, ergotamine)
• lovastatin
• simvastatin
• rosuvastatin

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• diarrhea
• nausea
• vomiting

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• rapid heartbeat
• symptoms of irregular heartbeat (e.g., chest pain, dizziness, rapid, pounding heartbeat, shortness of breath)

Stop taking the medication and seek immediate medical attention if any of the following occur:

• signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Kidney function: Decreased kidney function or kidney disease may cause this medication to build up in your body, causing side effects. If you have kidney problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Liver function: Decreased liver function or liver disease may cause this medication to build up in your body, causing side effects. If you have liver problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Pregnancy: This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: It is not known if letermovir passes into breast milk. If you are breast-feeding and taking this medication, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The safety and effectiveness of using this medication have not been established for children.

There may be an interaction between letermovir and any of the following:

• alitretinoin
• alpha blockers (e.g., alfuzosin, silodosin, tamsulosin)
• antiarrhythmics (e.g., amiodarone, disopyramide, dronedarone)
• anti-cancer medications (e.g., docetaxel, doxorubicin; ifosfamide, irinotecan, vincristine)
• antipsychotics (e.g., aripiprazole, cariprazine, clozapine, pimozide, quetiapine)
• apalutamide
• apixaban
• aprepitant
• atogepant
• avacopan
• "azole" antifungals (e.g., itraconazole, voriconazole)
• benzodiazepines (e.g., alprazolam, diazepam, triazolam)
• birth control pills
• bosentan
• bromocriptine
• buprenorphine
• buspirone
• calcium channel blockers (e.g., amlodipine, diltiazem, nifedipine, verapamil)
• cannabis
• CF transmembrane regulators (e.g., ivacaftor, lumacaftor, tezacaftor)
• clindamycin
• colchicine
• corticosteroids (e.g., budesonide, dexamethasone, fluticasone, methylprednisolone)
• cyclosporine
• darifenacin
• darolutamide
• domperidone
• eletriptan
• eliglustat
• eltrombopag
• eplerenone
• ergot alkaloids (e.g., dihydroergotamine, ergonovine, ergotamine, methylergonovine)
• erythromycin
• everolimus
• finerenone
• flibanserin
• gemfibrozil
• glyburide
• guanfacine
• HIV non-nucleoside reverse transcriptase inhibitors (NNRTIs; e.g., efavirenz, etravirine, nevirapine)
• HIV protease inhibitors (e.g., atazanavir, ritonavir, tipranavir)
• ivabradine
• leflunomide
• lemborexant
• lidocaine
• lomitapide
• macitentan
• maraviroc
• mavacamten
• methadone
• mifepristone
• modafinil
• naloxegol
• narcotic pain relievers (e.g., codeine, fentanyl, hydrocodone, oxycodone, tramadol)
• nirmatrelvir and ritonavir
• phenobarbital
• phosphodiesterase 5 inhibitors (e.g., sildenafil, tadalafil, vardenafil)
• pimecrolimus
• praziquantel
• protein kinase inhibitors (e.g., bosutinib, lapatinib, nilotinib, pazopanib)
• quinine
• ranolazine
• repaglinide
• rifabutin
• rifampin
• rivaroxaban
• roflumilast
• rosiglitazone
• St. John’s wort
• salmeterol
• saxagliptin
• seizure medications (e.g., carbamazepine, phenytoin, primidone)
• sirolimus
• solifenacin
• "statin" anti-cholesterol medications (e.g., atorvastatin, rosuvastatin, simvastatin)
• tacrolimus
• temsirolimus
• teriflunomide
• ticagrelor
• tofacitinib
• tolterodine
• tolvaptan
• trazodone
• venetoclax
• vilazodone
• warfarin
• zopiclone

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.