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Overview: Foscarnet is used to treat a viral eye infection called cytomegalovirus (CMV) retinitis in people with AIDS. This medicine is also used for certain herpes simplex virus infections in people with weakened immune systems. Foscarnet prevents cytomegalovirus and herpes simplex virus from multiplying in the body. Common side effects include kidney damage, electrolyte changes, seizures, fever, stomach upset, headache, and bone marrow suppression. Keep all appointments to have your blood checked. Contact your health care provider right away if you have a reduced need to pee, weakness or unusual tiredness, muscle or nerve problems, seizures, changes in heart rate or rhythm, fever, or easy bruising or bleeding.

Uses

What is foscarnet used for?

Foscarnet is commonly used to treat cytomegalovirus (CMV) retinitis, a viral eye infection, in people with AIDS and certain herpes simplex virus (HSV) infections in people with weakened immune systems.

Foscarnet may not be appropriate for some of the infections listed above, depending on which germs are causing the infection. Viruses in some locations may have developed resistance to foscarnet, which will make it not work as well.

Foscarnet may also be used for other conditions as determined by your health care provider.

How does foscarnet work (mechanism of action)?

Foscarnet prevents both cytomegalovirus and herpes simplex virus from multiplying in the body by disrupting their ability to create DNA.

How is foscarnet supplied (dosage forms)?

Brands and other names

  • Foscavir

Dosage forms and strengths

Foscarnet is available in an injectable form that is given by a health care provider.

How should I store foscarnet?

Foscarnet is usually given by a health care provider in a hospital or clinic. If you will store foscarnet at home, follow the instructions given by your health care provider.

Side Effects

What are the most common side effects of foscarnet?

The most common side effects of foscarnet are listed below. Tell your health care provider if you have any of these side effects that bother you.

  • Kidney damage (see below)
  • Electrolyte changes (see below)
  • Seizures (see below)
  • Fever
  • Nausea and vomiting
  • Diarrhea
  • Headache
  • Bone marrow suppression (see below)

There may be other side effects of foscarnet that are not listed here. Contact your health care provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088. In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of foscarnet?

While less common, the most serious side effects of foscarnet are described below, along with what to do if they happen.

Severe Allergic Reactions. Foscarnet may cause allergic reactions, which can be serious. Stop using foscarnet and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching, skin rash, or pale red bumps on the skin called hives
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Kidney Damage. Kidney damage can happen when using foscarnet. Keep all appointments, including lab appointments. Your health care provider needs bloodwork to make sure that you can safely take this medicine. Call your health care provider right away if you have any of the following symptoms of kidney damage.

  • Reduced need to pee
  • Swelling in your feet, ankles, or legs
  • Weakness or unusual tiredness
  • Difficulty catching your breath or chest pain/pressure
  • Confusion
  • Nausea
  • Seizures

Electrolyte Changes. Foscarnet may cause changes to the levels of certain minerals in your body, called electrolytes. For example, it may cause low calcium levels (hypocalcemia), low phosphate levels (hypophosphatemia) or high phosphate levels (hyperphosphatemia), low magnesium levels (hypomagnesemia), or low potassium levels (hypokalemia). These changes can be serious. Keep appointments for blood work, so your health care provider can check your electrolyte levels, especially if you take certain other medicines. Call your health care provider if you have any of the following symptoms of electrolyte changes.

  • Weakness
  • Unusual tiredness
  • Muscle cramps or twitching
  • Constipation
  • Nausea, vomiting, or loss of appetite
  • Dizziness
  • Restlessness or irritability
  • Difficulty breathing
  • Confusion or decreased alertness
  • Numbness, tingling, or burning in your arms, feet, or face
  • Seizures
  • Abnormal heart rhythm

Seizures. Some people who have used foscarnet have reported having seizures. There are many different types of seizures and some of them are not easy to recognize. Stop using foscarnet and call your health care provider if you have any of the following signs that a seizure is happening or could happen.

  • Muscle twitching, jerking, spasms, tremors, or other uncontrolled movements
  • Stiffening of your arms and legs
  • Feeling anxious, nervous, confused, or a sense of dread
  • Feeling dizzy or lightheaded
  • Falling or losing consciousness

Heart Rhythm Changes. Foscarnet may cause a rare, dangerous heart rhythm problem called QT prolongation and torsade de pointes. Some people have a higher risk of this, including people who are older, those who have other people in their family who have had these conditions, those with low potassium or magnesium levels, or those who take medicines that can prolong the QT interval. Tell your health care provider right away if you faint or have changes in your heart rate or rhythm, such as a fast or skipping heartbeat.

Bone Marrow Suppression. Foscarnet may cause your bone marrow to not make enough cells. This can lead to low blood levels of important cells, such as red blood cells, white blood cells, platelets, and neutrophils. Bone marrow suppression can lead to infections or bleeding, which can be serious or even life-threatening. Keep all appointments to have your blood checked. Tell your health care provider right away if you have any of the following symptoms of bone marrow suppression.

  • Fever
  • Shortness of breath
  • Pale or yellowish skin
  • Easy bruising or bleeding
  • Frequent infection
  • Unusual weakness or tiredness
  • Dizziness, lightheadedness, or feeling like you are about to pass out
  • Headache
  • Fast or abnormal heartbeat

Warnings & Precautions

Who should not use foscarnet?

Allergies to Ingredients. People who are allergic to any of the following should not use foscarnet.

  • Foscarnet
  • Foscavir
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific foscarnet products they stock.

What should I know about foscarnet before using it?

Foscarnet is usually given by a health care provider in a hospital or clinic.

You should stay well hydrated by drinking plenty of water while using foscarnet.

Keep all follow up-appointments while you are being treated with foscarnet. If you are being treated for an eye infection, your health care provider may need to check your eyes regularly.

Foscarnet can affect your alertness or coordination. Some people may have dizziness. Do not drive or do other activities that require alertness or coordination until you know how foscarnet affects you.

What should I tell my health care provider before using foscarnet?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them determine if foscarnet is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your health care provider if you have any of the following.

Other Medicines and Supplements. Foscarnet may interact with other medicines and supplements. Before using foscarnet, tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. It is not known if or how foscarnet could affect pregnancy or harm an unborn baby. Tell your health care provider if you are pregnant or plan to become pregnant. Your health care provider will let you know if you should use foscarnet while you are pregnant or trying to get pregnant.

Breastfeeding. It is not known if foscarnet passes into breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed. Your health care provider will let you know if you should stop breastfeeding or stop foscarnet.

Interactions

Does foscarnet interact with foods or drinks?

There are no known interactions between foscarnet and foods or drinks.

There are no known interactions between foscarnet and alcohol, but heavy drinking can slow your immune response and make it harder for your body to fight infection.

Does foscarnet interact with other medicines (drug interactions)?

Always tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before using foscarnet.

  • Acyclovir, which is used to treat certain viral infections
  • Amphotericin B (Abelcet, AmBisome), which is a medicine used to treat fungal infections
  • Cyclosporine (Gengraf, Neoral, Sandimmune, and others) or tacrolimus (Astagraf XL, Envarsus XR, Prograf), which are used to suppress your immune system
  • Intravenous pentamidine (Pentam), which is used to treat Pneumocystis pneumonia
  • Methotrexate (Trexall and others), which is commonly used for rheumatoid arthritis or certain types of cancer
  • Ritonavir (Norvir), which is used to treat HIV
  • An aminoglycoside antibiotic, such as amikacin, gentamicin, or tobramycin, which are used to treat various infections
  • A phenothiazine, such as chlorpromazine or prochlorperazine (Compro and others), which may be used to treat certain mental health conditions or severe nausea and vomiting
  • A tricyclic antidepressant, such as amitriptyline, nortriptyline, or protriptyline, even if these are not used for depression
  • A diuretic, also called a water pill, which is used to reduce edema (fluid retention) and blood pressure
  • Any medicine that can cause QT prolongation, which is a potentially dangerous heart rhythm problem, such as quinidine, procainamide, dofetilide, amiodarone, or sotalol

This may not be a complete list of medicines that can interact with foscarnet. Always check with your health care provider.

Does interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose/Missed Dose

What should I do if I accidentally use too much foscarnet?

If you or someone else has used too much foscarnet, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of foscarnet?

Missing doses of foscarnet can cause it to not work as well over time. For this reason, it is important not to miss or skip doses.

Foscarnet is usually given by a health care provider in a hospital or clinic. Your health care provider will make sure you are receiving foscarnet as scheduled.

If you miss an appointment to get your foscarnet dose, call to make another appointment as soon as possible.