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 Prevymis 240. If you are concerned about side effects, discuss the risks and benefits of Prevymis 240 with your doctor.

The following side effects have been reported by at least 1% of people taking Prevymis 240. 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 Prevymis 240.

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 Prevymis 240 if you:

• are allergic to letermovir or any ingredients of the medication
• are taking the medication Pimozide
• are taking ergotamine or Dihydroergotamine for migraines

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

• alpha blockers (e.g., Alfuzosin, Doxazosin, Silodosin, Tamsulosin)
• antiarrythmics (e.g., Amiodarone, Disopyramide, dofetilide, quinidine)
• anti-cancer medications (e.g., Cabazitaxel, Docetaxel, Doxorubicin; Etoposide, Ifosfamide, Irinotecan, Vincristine)
• antipsychotics (e.g., Aripiprazole, Clozapine, Haloperidol, Olanzapine, pimozide, Quetiapine, Risperidone)
• Apixaban
• Aprepitant
• "azole" antifungals (e.g., itraconazole, ketoconazole, Voriconazole)
• benzodiazepines (e.g., Chlordiazepoxide, Clonazepam, Diazepam, Lorazepam)
• Bosentan
• Bromocriptine
• Buspirone
• Calcitriol
• calcium channel blockers (e.g., Amlodipine, Diltiazem, Nifedipine, Verapamil)
• cannabis
• chloroquine
• Colchicine
• conivaptan
• corticosteroids (e.g., budesonide, Dexamethasone)
• Cyclosporine
• Dapsone
• Darifenacin
• Domperidone
• Eletriptan
• Eltrombopag
• Eplerenone
• ergot alkaloids (e.g., dihydroergotamine, ergonovine, ergotamine, methylergonovine)
• Everolimus
• Gemfibrozil
• "gliptin" Diabetes medications (e.g., Linagliptin, Saxagliptin, Sitagliptin)
• Glyburide
• Guanfacine
• Hepatitis C antivirals (e.g., asunaprevir, daclatasvir, grazoprevir, simeprevir)
• HIV non-nucleoside reverse transcriptase inhibitors (NNRTIs; e.g., delaviridine, efavirenz, etravirine, nevirapine)
• HIV protease inhibitors (e.g., atazanavir, indinavir, ritonavir, saquinavir)
• Hydroxyzine
• Ivacaftor
• lidocaine
• Lomitapide
• macrolide antibiotics (e.g., clarithromycin, erythromycin)
• maraviroc
• Mefloquine
• Methadone
• mifepristone
• Mirtazapine
• Naloxegol
• narcotic pain relievers (e.g., Fentanyl, Hydrocodone, Oxycodone)
• nefazodone
• nitrates (e.g., isosorbide dinitrate, isosorbide mononitrate)
• Pimecrolimus
• protein kinase inhbitors (e.g., Bosutinib, imatinib, Lapatinib, Nilotinib, Pazopanib
• quinine
• Repaglinide
• rifabutin
• rifampin
• Rosiglitazone
• salmeterol
• saxagliptin
• seizure medications (e.g., Carbamazepine, Clobazam, Levetiracetam, Phenobarbital, Phenytoin, primidone, Topiramate, Valproic acid, zonisamide)
• selective serotonin reuptake inhibitors (SSRIs; e.g., Citalopram, , Fluoxetine, Paroxetine, Sertraline)
• Sildenafil
• "statin" anti-cholesterol medications (e.g., Atorvastatin, Lovastatin, Simvastatin)
• Sirolimus
• Tacrolimus
• Tamoxifen
• tamsulosin
• Teriflunomide
• Ticagrelor
• Tolterodine
• Tolvaptan
• Trazodone
• tricyclic antidepressants (e.g., Amitriptyline, Clomipramine, Desipramine, Trimipramine)
• Ulipristal
• Venlafaxine
• 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 Prevymis 240. 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.

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 Prevymis 240.

Kidney function: Decreased kidney function or Kidney Disease may cause Prevymis 240 to build up in your body, causing side effects. If you have kidney problems, discuss with your doctor how Prevymis 240 may affect your medical condition, how your medical condition may affect the dosing and effectiveness of Prevymis 240, and whether any special monitoring is needed.

Liver function: Decreased liver function or liver disease may cause Prevymis 240 to build up in your body, causing side effects. If you have liver problems, discuss with your doctor how Prevymis 240 may affect your medical condition, how your medical condition may affect the dosing and effectiveness of Prevymis 240, and whether any special monitoring is needed.

Pregnancy: Prevymis 240 should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking Prevymis 240, contact your doctor immediately.

Breast-feeding: It is not known if letermovir passes into breast milk. If you are a breast-feeding mother and are taking Prevymis 240, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The safety and effectiveness of using Prevymis 240 have not been established for children.