Acyclovir is a commonly used antiviral medication that works to inhibit viral DNA replication by acting as a nucleoside analogue that mimics guanosine, a purine based guanine attached to a ribose sugar.  In the hospital setting, it is sometimes used intravenously to cover for known or suspected Herpes Meningitis.  Acyclovir has a low solubility and, when used intravenously, can crystalize out in the renal tubules causing renal tubular inflammation and obstruction, especially in those who are already volume depleted.  Symptoms, if any, may include nausea and flank pain.  Creatinine will usually bump within 24-48 hours.  Discontinuation of Acyclovir and IV hydration is the usual treatment, often with good result.  Those with Chronic Renal Insufficiency are particularly, but not exclusively, at risk.  Oral Acyclovir has also been implicated in renal dysfunction but to a much lesser extent than IV Acyclovir, likely secondary to the rapid excretion of IV Acyclovir in the kidneys which saturates the tubules.  It is important to follow the Creatinine level closely during intravenous Acyclovir use, especially in the first 48 hours, in all patients.