Acyclovir-induced colitis.

Three patients developed acute colitis, either de novo, or as an exacerbation of pre-existing colitis, following the use of oral acyclovir, prescribed for Herpes zoster or Herpes simplex infection. Rechallenge with oral acyclovir was performed in one patient, and resulted in a recurrence of colitic symptoms. It is speculated that acyclovir can have a direct irritant effect on large bowel mucosa.