Before using foscarnet, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of: liver disease, kidney disease (including being on dialysis), dehydration, low electrolyte levels (calcium, magnesium, potassium, phosphate), nervous system problems (such as a history of seizures), a low sodium diet.

This drug may make you dizzy. Alcohol or marijuana (cannabis) can make you more dizzy. Do not drive, use machinery, or do anything that needs alertness until you can do it safely. Limit alcoholic beverages. Talk to your doctor if you are using marijuana (cannabis).

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

Children may be more sensitive to the side effects of this drug, especially the effects on developing bones.

Older adults may be more sensitive to the side effects of this drug, especially kidney problems.

During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor.

It is unknown if this drug passes into breast milk. Because of the possible risk to the infant, breastfeeding is not recommended while using this drug. Consult your doctor before breastfeeding.