Does acyclovir increase serum lithium levels?

A 42-year-old woman was admitted to the hospital to receive intravenous acyclovir for a herpes zoster infection. At the time she was taking lithium carbonate 450 mg twice/day. Six days after starting acyclovir she exhibited signs of lithium toxicity. When measured, the serum lithium level had increased 4-fold during acyclovir therapy. Both agents are excreted by the kidneys, raising the possibility that acyclovir at high serum concentrations may interfere with the renal excretion of lithium. A MEDLINE search did not identify any citation describing the possibility of an interaction between the drugs. This case suggests that acyclovir when given intravenously in doses of 10 mg/kg may result in increased serum lithium concentrations. Until additional data are available, if intravenous acyclovir is administered concurrently with lithium, we recommend closely monitoring patients for signs of lithium toxicity and measuring serum lithium levels every second or third day.