Valtrex (valacyclovir) is commonly prescribed for patients with herpes infections - herpes zoster or genital herpes. This drug should be taken regularly for you to benefit from the treatment. Tell your health care provider if you have any of the following medical conditions before starting to take Valtrex: human immunodeficiency virus infection, liver disease, acquired immunodeficiency syndrome, problems with your immune system, or kidney disease. Those medical conditions may be important in determining the dose you are supposed take and need to be discussed with your doctor very carefully. Also, let your health care provider know if you are currently taking lithium, methotrexate, medicines used to treat ulcerative colitis, antiviral medicines, pain or arthritis medicines, IV antibiotics, cancer medicines, drugs used to prevent organ transplant rejection, or any other prescription or over-the-counter products. Interactions with Valtrex are possible but can be avoided if everything is discussed in advance. If you get vomiting, diarrhea or loose stools, constipation, upset stomach, or headache - continue the treatment, as those are mild side effects of Valtrex.