Fostemsavir is used along with other medications to treat human immunodeficiency virus (HIV) infection in adults whose HIV could not be successfully treated with other medications, including their current therapy. Fostemsavir is in a class of medications called HIV attachment inhibitors. It works by decreasing the amount of HIV in the blood. Although fostemsavir does not cure HIV, it may decrease your chance of developing acquired immunodeficiency syndrome (AIDS) and HIV-related illnesses such as serious infections or cancer. Taking these medications along with practicing safer sex and making other life-style changes may decrease the risk of transmitting (spreading) the HIV virus to other people.

Side Effects Of Fostemsavir

Fostemsavir may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away:

  • nausea
  • vomiting
  • diarrhea
  • stomach pain
  • indigestion
  • headache
  • tiredness
  • rash
  • difficulty falling asleep or staying asleep

Some side effects can be serious. If you experience any of these symptoms call your doctor immediately:

  • dizziness, lightheadedness, fainting, or irregular heartbeat

Fostemsavir may cause other side effects. Call your doctor if you have any unusual problems while taking this medication.

Warnings & Precautions

Before taking fostemsavir:

  • tell your doctor and pharmacist if you are allergic to fostemsavir, any other medications, or any of the ingredients in fostemsavir tablets. Ask your pharmacist for a list of the ingredients.
  • tell your doctor if you are taking any of the following medications: carbamazepine (Carbatrol, Epitol, Equetro, Tegretol, Teril), enzalutmide (Xtandi), mitotane (Lysodren), phenytoin (Dilantin, Phenytek), rifampin (Rifadin, Rimactane, in Rifamate, in Rifater), or St. John’s Wort. Your doctor will probably tell you not to take fostemsavir if you are taking one or more of these medications.
  • tell your doctor and pharmacist what other prescription and nonprescription medications, vitamins, nutritional supplements, and herbal products you are taking or plan to take. Be sure to mention: atorvastatin (Lipitor, in Caduet), fluvastatin (Lescol), grazoprevir (in Zepatier), hormonal contraceptives (birth control pills, patches, rings, or injections), pitivastatin (Livalo, Zypitamag), rosuvastatin (Crestor, Ezallor), simvastatin (Flolipid, Zocor, in Vytorin), or voxilaprevir (in Vosevi). Your doctor may need to change the doses of your medications or monitor you carefully for side effects. Many other medications may also interact with fostemsavir, so be sure to tell your doctor about all the medications you are taking, even those that do not appear on this list.
  • tell your doctor if you have or have ever had prolonged QT interval (a rare heart problem that may cause fainting or irregular heartbeat), irregular heartbeat, other heart problems. Also, tell your doctor if you have or have ever had hepatitis (a viral infection of the liver) or any other liver disease.
  • tell your doctor if you are pregnant, plan to become pregnant, or are breastfeeding. If you become pregnant while taking fostemsavir, call your doctor. You should not breastfeed if you are infected with HIV or are taking fostemsavir.
  • you should know that while you are taking medications to treat HIV infection, your immune system may get stronger and begin to fight other infections that were already in your body. This may cause you to develop symptoms of those infections. If you have new or worsening symptoms after starting treatment with fostemsavir, be sure to tell your doctor.

Fostemsavir Dosage

Fostemsavir comes as a tablet to take by mouth with or without food. It is usually taken twice a day. Take fostemsavir at around the same times every day. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take fostemsavir exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.

Swallow the tablets whole; do not split, chew, or crush them.

Fostemsavir controls HIV but does not cure it. Continue to take fostemsavir even if you feel well. Do not stop taking fostemsavir without talking to your doctor. If you miss doses or stop using fostemsavir, your condition may become more difficult to treat.

Ask your pharmacist or doctor for a copy of the manufacturer’s information for the patient.


Keep all appointments with your doctor and the laboratory. Your doctor will order certain lab tests to check your body’s response to fostemsavir.

Do not let anyone else take your medication. Ask your pharmacist any questions you have about refilling your prescription.

Fostemsavir tablets may have a slight vinegar-like odor. Ask your pharmacist if you have any questions regarding your medication.

It is important for you to keep a written list of all of the prescription and nonprescription (over-the-counter) medicines you are taking, as well as any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a doctor or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.


All information has been provided courtesy of MedLinePlus from the National Library of Medicine and from the FDA.