Praziquantel, the active ingredient, is a pyrazinoisoquinoline derivative used widely as an anthelmintic for both human and veterinary medicine. The chemical name for this substance is 2-cyclohexyl-carbonyl[1,2,3,6,7,11b] hexahydro-4H-pyrazino-[2,1-a]isoquinolin-4-one. Praziquantel is effective against all stages of development of intestinal tapeworms. The substance is very rapidly absorbed and distributed throughout the parasite. Both in vivo and in vitro studies have shown that praziquantel causes severe damage to the parasite integument, resulting in contraction and paralysis. There is an almost instantaneous tetanic contraction of the parasite musculature and a rapid vacuolisation of the syncytial tegument. This rapid contraction has been explained by changes in divalent cation fluxes, particularly calcium.