• you are allergic to any ingredient in Acivir or to valacyclovir.

Some medical conditions may interact with Acivir. Tell your doctor or pharmacist if you have any medical conditions, especially if any of the following apply to you:

• if you are pregnant, planning to become pregnant, or are breast-feeding
• if you are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
• if you have allergies to medicines, foods, or other substances
• if you have kidney problems or a weakened immune system.

Some medicines may interact with Acivir. Tell your health care provider if you are taking any other medicines, especially any of the following:

• Medicines that may harm the kidney (eg, aminoglycoside antibiotics [eg, gentamicin], amphotericin B, cyclosporine, nonsteroidal anti-inflammatory drugs [NSAIDs] [eg, ibuprofen], tacrolimus, vancomycin) because the risk of kidney side effects may be increased. Ask your doctor if you are unsure if any of your medicines might harm the kidney.

This may not be a complete list of all interactions that may occur. Ask your health care provider if Acivir may interact with other medicines that you take. Check with your health care provider before you start, stop, or change the dose of any medicine.

Important safety information:

• Acivir may cause drowsiness, dizziness, or changes in vision. These effects may be worse if you take it with alcohol or certain medicines. Use Acivir with caution. Do not drive or perform other possible unsafe tasks until you know how you react to it.
• Acivir may cause dizziness, lightheadedness, or fainting; alcohol, hot weather, exercise, or fever may increase these effects. To prevent them, sit up or stand slowly, especially in the morning. Sit or lie down at the first sign of any of these effects.
• Acivir may cause you to become sunburned more easily. Avoid the sun, sunlamps, or tanning booths until you know how you react to Acivir. Use a sunscreen or wear protective clothing if you must be outside for more than a short time.
• Acivir does not eliminate the virus and is not a cure. It does not prevent transmission of the virus to others.
• Acivir is not a cure for herpes simplex infections. Avoid sexual intercourse when visible lesions are present to prevent infecting your partner.
• Use Acivir with caution in the elderly; they may be more sensitive to its effects, especially confusion, drowsiness, or hallucinations.
• Pregnancy and breast-feeding: If you become pregnant, contact your doctor. You will need to discuss the benefits and risks of using Acivir while you are pregnant. Acivir is found in breast milk. If you are or will be breast-feeding while you use Acivir, check with your doctor. Discuss any possible risks to your baby.

Check with your doctor if any of these most common side effects persist or become bothersome:

Diarrhea; general body discomfort; headache; nausea/vomiting.

Seek medical attention right away if any of these severe side effects occur:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); aggressive behavior; blood in the urine; confusion; decreased consciousness; decreased urination; hallucinations; lower back pain; mental or mood changes; red, swollen, blistered, or peeling skin; seizures; unusual bruising or bleeding.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects.