In the U.S.—

Other commonly used names are phosphonoformic acid; PFA.

• Antiviral, systemic

Foscarnet (foss-KAR-net) is used to treat the symptoms of cytomegalovirus (CMV) infection of the eyes in patients with acquired immune deficiency syndrome (AIDS). Foscarnet will not cure this eye infection, but it may help to control worsening of the symptoms. It is also used to treat herpes simplex virus (HSV) infections of the skin and mucous membranes in people who are immunocompromised and whose infections did not improve with other therapy. Foscarnet may also be used for other serious viral infections as determined by your doctor. However, it does not work in treating certain viruses, such as the common cold or the flu.

Foscarnet is administered only by or under the supervision of your doctor. It is available in the following dosage form:

Parenteral
• Injection (U.S.)

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For foscarnet, the following should be considered:

Allergies—Tell your doctor if you have ever had any unusual or allergic reaction to foscarnet. Also tell your health care professional if you are allergic to any other substances, such as foods, preservatives, or dyes.

Pregnancy—Foscarnet has not been studied in pregnant women. However, studies in animals have shown that foscarnet causes birth defects. Before taking this medicine, make sure your doctor knows if you are pregnant or if you may become pregnant.

Breast-feeding—It is not known whether foscarnet passes into the breast milk. Although most medicines pass into breast milk in small amounts, many of them may be used safely while breast-feeding. Mothers who are taking this medicine and who wish to breast-feed should discuss this with their doctor.

Children—There is no specific information comparing use of foscarnet in children with use in other age groups. Foscarnet can cause serious side effects in any patient. Therefore, it is especially important that you discuss with the child's doctor the good that this medicine may do as well as the risks of using it.

Older adults—Many medicines have not been studied specifically in older people. Therefore, it may not be known whether they work exactly the same way they do in younger adults or if they cause different side effects or problems in older people. There is no specific information comparing use of foscarnet in the elderly with use in other age groups.

Other medicines—Although certain medicines should not be used together at all, in other cases 2 different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking foscarnet, it is especially important that your health care professional know if you are taking any of the following:

• Carmustine (e.g., BiCNU) or
• Cisplatin (e.g., Platinol) or
• Combination pain medicine containing acetaminophen and aspirin (e.g., Excedrin) or other salicylates (with large amounts taken regularly) or
• Cyclosporine (e.g., Sandimmune) or
• Deferoxamine (e.g., Desferal) (with long-term use) or
• Gold salts (medicine for arthritis) or
• Inflammation or pain medicine, except narcotics, or
• Lithium (e.g., Lithane) or
• Methotrexate (e.g., Mexate) or
• Other anti-infectives (e.g., amphotericin B) or
• Penicillamine (e.g., Cupramine) or
• Plicamycin (e.g., Mithracin) or
• Streptozocin (e.g., Zanosar) or
• Tiopronin (e.g., Thiola)—Use of these medicines may increase the chance of side effects affecting the kidneys

• Pentamidine (e.g., Pentam)—Use of pentamidine injection with foscarnet may lower the level of important minerals (calcium and magnesium) in the blood; it may also increase the chance of side effects affecting the kidneys

• Anemia—Foscarnet may cause or worsen anemia

• Dehydration or
• Kidney disease—Patients who are dehydrated or have kidney disease may have an increased chance of side effects

To ensure the best response, foscarnet must be given for the full time of treatment. Also, this medicine works best when there is a constant amount in the blood. To help keep the amount constant, foscarnet must be given on a regular schedule.

Several glasses of water should be taken every day, unless otherwise directed by your doctor. Drinking extra water will help to prevent some unwanted effects foscarnet has on the kidneys.

This medicine may cause sores on the genitals (sex organs). Washing your genitals after urination may decrease the chance of your developing this problem.

Dosing—

The dose of foscarnet will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of foscarnet. If your dose is different, do not change it unless your doctor tells you to do so.

• For injection dosage form:
  • For cytomegalovirus (CMV) retinitis induction (first stage of dosing):
    • Adults and children—The usual dose is 60 milligrams (mg) per kilogram (kg) (27.3 mg per pound) of body weight every eight hours for fourteen to twenty-one days. Each dose is injected slowly into a vein by an infusion pump over at least one hour.
  • For CMV retinitis maintenance (second stage of dosing):
    • Adults and children—The usual dose is 90 to 120 mg per kg (41 to 54.5 mg per pound) of body weight once a day. This dose is injected slowly into a vein by an infusion pump over at least two hours.
  • For herpes simplex infections:
    • Adults and children—The usual dose is 40 mg per kg (18.2 mg per pound) of body weight given either every eight or every twelve hours. This dose is injected slowly into a vein by an infusion pump over at least one hour. Treatment is usually continued for two to three weeks or until the infection in healed.

It is very important that your doctor check your progress at regular visits. This will allow your doctor to check for possible unwanted effects.

It is also very important that your ophthalmologist (eye doctor) check your eyes at regular visits since you may have some loss of eyesight due to retinitis even while you are receiving foscarnet.

Along with their needed effects, medicines like foscarnet can sometimes cause serious side effects such as kidney problems; these are described below. Foscarnet may also decrease the amount of calcium in your blood, causing you to have a tingling sensation around your mouth, and pain or numbness in your hands and feet. If this occurs, especially while you are receiving the medicine, notify your health care professional immediately.

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur:

• More common
  • Increased or decreased frequency of urination or amount of urine ;  increased thirst 
  
  

• Less common
  • Convulsions (seizures);  fever, chills, and sore throat;  muscle twitching;  pain at place of injection;  pain or numbness in hands or feet;  tingling sensation around mouth;  tremor;  unusual tiredness and weakness  
  
  

• Rare
  • Sores or ulcers on the mouth or throat, penis, or vulva 
  
  

Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. However, check with your doctor if any of the following side effects continue or are bothersome:

• More common
  • Abdominal or stomach pain;  anxious feeling ;  confusion;  dizziness;  headache;  loss of appetite;  nausea and vomiting;  unusual tiredness or weakness 
  
  

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor.

Once a medicine has been approved for marketing for a certain use, experience may show that it is also useful for other medical problems. Although these uses are not included in product labeling, foscarnet is used in certain patients with the following medical conditions:

• Cytomegalovirus infections in places other than the eyes, such as the lungs, esophagus, or intestines
• Varicella-zoster infection (shingles) that does not respond to treatment with acyclovir in patients with HIV infection

Other than the above information, there is no additional information relating to proper use, precautions, or side effects for these uses.