Anthelmintics or antihelminthics are drugs that expel parasiticworms (helminths) and other internal parasites from the body by either stunning or killing them and without causing significant damage to the host. They may also be called vermifuges (those that stun) or vermicides (those that kill). They are used to treat people or animals who are infected by helminths - a condition called helminthiasis.
The ability of parasites to survive treatments that are generally effective at the recommended dose rate is a major threat to the future control of worm parasites in small ruminants and horses. This is especially true of nematodes, and has helped spur development of aminoacetonitrile derivatives for treatment against drug resistant nematodes.
The clinical definition of resistance is a 95% or less reduction in a "Fecal Egg Count" test.[clarification needed]
Treatment with an antihelminthic drug kills worms whose phenotype renders them susceptible to the drug. But resistant parasites survive and pass on their "resistance" genes. Resistant varieties accumulate and finally treatment failure occurs. See drug resistance.