The Collaborative International Dictionary of English v.0.48:

Diarrhetic \Di`ar*rhet"ic\, Diarrhoetic \Di`ar*rh[oe]t"ic\, a.
   (Med.)
   Producing diarrhea, or a purging.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diarrhetic
    adj 1: of or relating to diarrhea [syn: diarrheal,
           diarrhoeal, diarrhetic, diarrhoetic, diarrheic,
           diarrhoeic]