Fostemsavir belongs to the class of medications called antiretrovirals. It is used in addition to other HIV medications to treat the infection caused by the human immunodeficiency virus (HIV) that has become resistant to multiple other medications used to treat HIV infection. It works by preventing the virus from attaching to cells, which then prevents the virus from infecting the cell and reproducing.

Fostemsavir does not cure AIDS and does not prevent it from being spread to others. It does slow further growth or reproduction of HIV when used in combination with other medications. It may help to delay the development of problems that are related to AIDS or HIV 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.

The recommended dose of fostemsavir is 600 mg (1 tablet) taken by mouth, 2 times daily. It is taken in combination with other anti-HIV medications.

Fostemsavir may be taken with or without food. Swallow the tablets whole with some water and do not crush or chew them.

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 at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Each beige, biconvex, oval, film-coated, extended-release tablet, debossed with "SV 1V7" on one side and plain on the other side, contains 600 mg of fostemsavir (as 724.56 mg of fostemsavir tromethamine). Nonmedicinal ingredients: colloidal silicon dioxide, hydroxypropyl cellulose, hypromellose, iron oxide red, iron oxide yellow, magnesium stearate, polyethylene glycol, polyvinyl alcohol, talc, and titanium dioxide.

Do not take this medication if you: 

• are allergic to fostemsavir or any ingredients of the medication
• are taking any of the following medications:
• carbamazepine
• enzalutamide
• fosphenytoin
• mitotane
• phenobarbital
• phenytoin
• primidone
• rifampin
• St. John's wort

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.

• abdominal pain
• diarrhea
• dizziness
• headache
• indigestion
• itching
• lack of energy
• muscle aches
• nausea
• rash
• sleepiness
• taste disturbance
• trouble sleeping
• unusual weakness
• 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:

• signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)
• symptoms of immune reconstitution inflammatory syndrome (e.g., fever, redness, rash or swelling, fatigue, joint or muscle pain, numbness or weakness in the extremities, pounding heartbeat, chest pain)
• symptoms of irregular heartbeat (e.g., chest pain, dizziness, rapid, pounding heartbeat, shortness of breath)
• tingling, numbness, or weakness in the hands and/or feet

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.

Heart rhythm: Fostemsavir can cause changes to the normal rhythm of the heart, including an irregular heartbeat called QT prolongation, when more than the recommended dose is taken. QT prolongation is a serious life-threatening condition that can cause fainting, seizures, and sudden death.

People with heart failure, angina, low potassium or magnesium levels, or those who are taking other medications that can cause changes to the heart rhythm, may be at risk of developing an irregular heartbeat. If you have heart problems, or you are taking other medications, 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.

Hepatitis B and C: People who have hepatitis B or hepatitis C may be more likely to experience decreased liver function and side effects involving the liver when they are taking fostemsavir. If you have hepatitis B or C, 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.

Immune reconstitution inflammatory syndrome: This medication may cause immune reconstitution syndrome, where signs and symptoms of inflammation from previous infections appear. These symptoms occur soon after starting anti-HIV medication and can vary. They are thought to occur because of the immune system improving and being able to fight infections that have been present without symptoms (such as pneumonia, herpes or tuberculosis). Report any new symptoms to your doctor immediately.

Stopping the medication: If you stop taking this medication, your HIV infection could get worse. Take the medication exactly as prescribed by your doctor and do not stop taking the medication without checking with your doctor first.

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 fostemsavir passes into breast milk.Women with HIV infection are cautioned against breast-feeding because of the risk of passing HIV to a baby who does not have the infection, since the virus can be transmitted through breast milk.

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

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

• anti-arrhythmic medications (e.g., amiodarone, disopyramide, dronedarone, procainamide, quinidine, sotalol)
• apalutamide
• chlorpromazine
• cladribine
• elagolix
• enzalutamide
• ethinyl estradiol
• haloperidol
• hepatitis C antivirals (e.g., glecaprevir, grazoprevir, ledipasvir, velpatasvir, voxilaprevir)
• HIV protease inhibitors (e.g., atazanavir, darunavir, ritonavir)
• methadone
• mitotane
• protein kinase inhibitors (e.g., alpelisib, pazopanib, vandetanib)
• quinine
• rifabutin
• rifampin
• St. John's wort
• seizure medications (e.g., carbamazepine, phenobarbital, phenytoin, primidone)
• "statin" anti-cholesterol medications (e.g., atorvastatin, lovastatin, simvastatin)
• topotecan
• ziprasidone

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.