Letermovir is an antiviral medicine used to help prevent cytomegalovirus (CMV) infection after a stem cell (bone marrow) transplant from a donor. Letermovir is for use in adults who are seropositive for CMV. Seropositive means that the virus is in your blood even if you do not show any symptoms of infection.

A CMV-seropositive person is at risk for "reactivation" of the virus if the immune system becomes weak. While normally treatable, CMV infection can be serious in people undergoing a stem cell transplant.

Letermovir may also be used for purposes not listed in this medication guide.

Tell your doctor about all your current medicines and any you start or stop using. Many drugs can interact with letermovir, and some drugs should not be used together.

Follow all directions on your prescription label and read all medication guides or instruction sheets. Use the medicine exactly as directed.

Letermovir oral is taken by mouth.

Letermovir injection is given as an infusion into a vein. A healthcare provider will give your first dose and may teach you how to properly use the medication by yourself.

You may take letermovir tablets with or without food.

Do not crush, chew, or break a letermovir tablet. Swallow it whole.

Read and carefully follow any Instructions for Use provided with your medicine. Ask your doctor or pharmacist if you don't understand all instructions.

Prepare an injection only when you are ready to give it. Do not use if the medicine looks cloudy, has changed colors, or has particles in it. Call your pharmacist for new medicine.

You may need to mix the injection solution with a liquid (diluent) in an IV bag before using it. If you are using the injections at home, be sure you understand how to properly mix and store the medicine.

You can store the mixture for up to 24 hours at room temperature, or up to 48 hours in a refrigerator.

Letermovir injection must be given slowly, and the infusion can take at least 1 hour to use all of the medicine in the IV bag.

Letermovir oral or injection is usually given once per day for up to 100 days after your stem cell transplant.

Use letermovir for the full prescribed length of time, even if you have no symptoms. Skipping doses may increase your risk of further infection that is resistant to medication.

Store letermovir tablets or unmixed injection solution in the original packaging at room temperature, away from moisture, heat, and light. Keep each tablet in the foil blister pack until you are ready to take a tablet.

Do not shake the injection solution bottle. Prepare your dose only when you are ready to give an injection. Do not use if the medicine has changed colors or has particles in it. Call your pharmacist for new medicine.

Each single-use vial (bottle) of letermovir injection is for one use only. Throw it away after one use, even if there is still medicine left inside.

Usual Adult Dose for CMV Prophylaxis:

480 mg orally or IV once a day
Duration of therapy: Through Day 100 posttransplantation

Comments:
-This drug should be started between Day 0 and Day 28 posttransplantation (before or after engraftment).

Use: For the prophylaxis of cytomegalovirus (CMV) infection and disease in CMV-seropositive recipients [R+] of an allogeneic hematopoietic stem cell transplant (HSCT)

Some medicines can cause unwanted or dangerous effects when used with letermovir. Your doctor may need to change your treatment plan if you use any of the following drugs:

• pimozide;

• cholesterol medication--pitavastatin, simvastatin; or

• ergot medicine--dihydroergotamine, ergotamine, ergonovine, methylergonovine.

Tell your doctor if you have ever had:

• kidney disease;

• liver disease;

• diabetes; or

• high cholesterol or triglycerides (a type of fat in the blood).

Tell your doctor if you are pregnant or breastfeeding.

Use the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not use two doses at one time.

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

Follow your doctor's instructions about any restrictions on food, beverages, or activity.

Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Letermovir may cause serious side effects. Call your doctor at once if you have:

• fast or irregular heartbeats.

Serious side effects may occur if you also use certain other medications.

Common side effects of letermovir may include:

• nausea, diarrhea, vomiting, stomach pain;

• swelling in your arms or legs;

• cough;

• headache; or

• feeling tired.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Sometimes it is not safe to use certain medications at the same time. Some drugs can affect your blood levels of other drugs you take, which may increase side effects or make the medications less effective.

Tell your doctor about all your current medicines and any you start or stop using. If you also use cyclosporine, your letermovir dose will need to be adjusted.

Other drugs may affect letermovir, including prescription and over-the-counter medicines, vitamins, and herbal products. Tell your doctor about all your current medicines and any medicine you start or stop using.