Foscarnet is an antiviral medicine that prevents certain viruses from multiplying in your body.

Foscarnet is used to treat cytomegalovirus (CMV) retinitis in people with AIDS.

Foscarnet is also used to treat herpes simplex virus (HSV) in people with a weak immune system. Foscarnet is usually given for HSV after other antiviral medications have been tried without successful treatment.

Foscarnet is not a cure for CMV or HSV, and your virus may progress during or after treatment.

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

Foscarnet can harm your kidneys. This effect is increased when you also use certain other medicines, including: antivirals, chemotherapy, injected antibiotics, medicine for bowel disorders, medicine to prevent organ transplant rejection, injectable osteoporosis medication, and some pain or arthritis medicines (including aspirin, Tylenol, Advil, and Aleve).

Your kidney function and electrolytes (potassium, sodium, magnesium, phosphorus) will need to be tested often.

Injecting foscarnet too fast can cause dangerous or unwanted side effects.

Follow all directions on your prescription label. Your doctor may occasionally change your dose. Do not use this medicine in larger or smaller amounts or for longer than recommended.

Foscarnet is injected into a vein through an IV using an infusion pump. The medicine enters the body through a catheter placed into the vein. A healthcare provider will show you how to use an infusion pump. Do not self-inject foscarnet if you do not understand how to give the injection and properly dispose of used needles, IV tubing, and other items used to inject the medicine.

You may also be given IV fluids to keep you from getting dehydrated.

You may need to mix foscarnet with a liquid (diluent) before using it. If you are using the injections at home, be sure you understand how to properly mix and store the medicine. Do not give foscarnet with other medicines in the same IV line.

Foscarnet should be clear and colorless. Gently shake the medicine if appears to have separated. Do not use the medicine if it has changed colors or has particles in it. Call your pharmacist for new medicine.

The medicine must be injected slowly and can take up to 2 hours to complete. Injecting foscarnet too fast can cause dangerous or unwanted side effects.

Foscarnet is usually given for 2 to 3 weeks. Follow your doctor's dosing instructions very carefully.

While using foscarnet, your kidney function and electrolytes (potassium, sodium, magnesium, phosphorus) will need to be tested often, and you may also need regular eye exams.

Store foscarnet at room temperature away from very hot or very cold temperature.

You should not use foscarnet if you are allergic to it.

To make sure foscarnet is safe for you, tell your doctor if you have:

• kidney disease;

• heart disease, heart rhythm disorder;

• personal or family history of long QT syndrome;

• an electrolyte imbalance (such as low levels of calcium, potassium, or magnesium in your blood);

• epilepsy or other seizure disorder; or

• if you are on a low salt diet.

It is not known whether foscarnet will harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant.

It is not known whether foscarnet passes into breast milk or if it could harm a nursing baby. You should not breast-feed while using this medicine.

Call your doctor for instructions if you miss a dose of foscarnet.

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

Avoid getting foscarnet in your eyes, as it may cause burning or irritation. If it does get into your eyes rinse with water and call your doctor.

This medicine may impair your thinking or reactions. Be careful if you drive or do anything that requires you to be alert.

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

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

• pain or burning when you urinate;

• a seizure (convulsions);

• headache with chest pain and severe dizziness, fainting, fast or pounding heartbeats;

• low white blood cell counts--fever, swollen gums, painful mouth sores, pain when swallowing, skin sores, cold or flu symptoms, cough, trouble breathing;

• low red blood cells (anemia)--pale skin, feeling light-headed or short of breath, rapid heart rate, trouble concentrating;

• low calcium--numbness or tingly feeling around your mouth, fast or slow heart rate, muscle tightness or contraction, overactive reflexes;

• low potassium--constipation, numbness or tingling, tiredness, muscle weakness, slow heart rate, fainting; or

• kidney problems--little or no urinating; painful or difficult urination; swelling in your feet or ankles; feeling tired or short of breath.

Some of foscarnet leaves the body in urine, which can cause irritation when you urinate. You may also develop sores or ulcers around your urethra (the opening where urine passes out of your bladder). Drink plenty of fluids and take care to keep your genital area clean while receiving this medicine.

Common side effects of foscarnet may include:

• fever, flu-like symptoms;

• nausea, vomiting, diarrhea;

• chest pain, back pain; or

• headache.

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.

Foscarnet can harm your kidneys. This effect is increased when you also use certain other medicines, including: antivirals, chemotherapy, injected antibiotics, medicine for bowel disorders, medicine to prevent organ transplant rejection, injectable osteoporosis medication, and some pain or arthritis medicines (including aspirin, Tylenol, Advil, and Aleve).

Tell your doctor about all your current medicines and any you start or stop using, especially:

• anagrelide, cilostazol, donepezil, fluconazole, methadone, ondansetron;

• an antibiotic or antifungal medicine--azithromycin, ciprofloxacin, clarithromycin, erythromycin, fluconazole, levofloxacin, moxifloxacin, pentamidine, telithromycin;

• cancer medicine--arsenic trioxide, oxaliplatin, vandetanib;

• an antidepressant--citalopram, escitalopram;

• anti-malaria medication--chloroquine, halofantrine;

• heart rhythm medicine--amiodarone, disopyramide, dofetilide, dronedarone, flecainide, ibutilide, procainamide, quinidine, sotalol; or

• medicine to treat a psychiatric disorder--chlorpromazine, droperidol, haloperidol, pimozide, thioridazine.

This list is not complete. Other drugs may interact with foscarnet, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed in this medication guide.