Valtrex (valacyclovir) is an efficient drug prescribed for infections caused by the herpes infection - such as herpes zoster and genital herpes. Taking Valtrex is not going to cure your herpes but it will make it possible for you to get the symptoms under control. Your doctor will need to know if you have acquired immunodeficiency syndrome, kidney disease, liver disease, problems with your immune system, or human immunodeficiency virus infection, as those conditions can affect the dose you are prescribed. Make sure you inform your doctor of the serious side effects in case you get any - such as blood in the urine, confusion, jaundice, rash, itching, or fever. Mild side effects are more likely and can include vomiting, upset stomach, diarrhea or loose stools, constipation, and headache. Do not start using any new medications until you have discussed that with your health care provider. The following ones have been reported to cause interactions with Valtrex: lithium, methotrexate, medicines used to treat ulcerative colitis, antiviral medicines, IV antibiotics, drugs used to prevent organ transplant rejection, cancer medicines, pain or arthritis medicines.