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