Anthelmintics or antihelminthics are a group of antiparasitic drugs that expel parasitic worms (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). Anthelmintics are used to treat people who are infected by helminths, a condition called helminthiasis. These drugs are also used to treat infected animals.
Pills containing anthelmintics are used in mass deworming campaigns of school-aged children in many developing countries. The drugs of choice for soil-transmitted helminths are mebendazole and albendazole; for schistosomiasis and tapeworms it is praziquantel.
Antiparasitics that specifically target worms of the genus Ascaris are called ascaricides.
Anthelmintic resistance in parasites is now widespread. It is a major threat to the sustainability of modern ruminant livestock production, resulting in reduced productivity, compromised animal health and welfare, and increased greenhouse gas emissions through increased parasitism and farm inputs. A database of published and unpublished European AR research on gastrointestinal nematodes was collated in 2020. A total of 197 publications were available for analysis, representing 535 studies in 22 countries and spanning the period 1980–2020. Results in sheep and goats since 2010 reveal an average prevalence of resistance to benzimidazoles of 86%, macrocyclic lactones except moxidectin 52%, levamisole 48%, and moxidectin 21%. All major gastrointestinal nematodes genera survived treatment in various studies. In cattle, prevalence of anthelminthic resistance varied between anthelmintic classes from 0–100% (benzimidazoles and macrocyclic lactones), 0–17% (levamisole) and 0–73% (moxidectin), and both Cooperia and Ostertagia survived treatment. 
The ability of parasites to survive treatments that are generally effective at the recommended doses 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, as well as exploration of doxycycline to kill their endosymbiotic Wolbachia bacteria.
The resistance is measured by the "fecal egg count reduction" value which varies for different types of helminths.
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 treatment failure finally occurs.
new anthelmintic alternatives such as tribendimidine and Nitazoxanide have proved to be safe and effective against A. lumbricoides and other soil-transmitted helminthiases in human trials.
Nitazoxanide, a 5-nitrothiazole derivative with broad-spectrum activity against helminths and protozoans, has been shown to be effective against C cayetanensis, with an efficacy 87% by the third dose (first, 71%; second 75%). Three percent of patients had minor side effects.