Triclabendazole, (brand name Avomec, Egaten, etc) is a member of the benzimidazole family of anthelmintics used to treat liver flukes, specifically fascioliasis and paragonimiasis. Triclabendazole used routinely since 1983 in veterinary practice for the treatment of fascioliasis. It was not used in humans until the 1989 epidemic of fascioliasis near the Caspian Sea when Iranian authorities approved the use of the veterinary formulation to treat the infection. Fasciolicidal not only against the adult worms present in the biliary ducts, but also against the immature larval stages of Fasciola migrating through the hepatic parenchyma. Triclabendazole is shown to penetrate into liver flukes by transtegumentary absorption followed by inhibition of the parasite's motility, probably related to the destruction of the microtubular structure, resulting in the death of the parasite; the immobilizing effect is paralleled by changes in the parasite's resting tegumental membrane potential, strongly inhibiting the release of proteolytic enzymes, a process that appears critical to the survival of the parasite. Side effects are generally few, but can include abdominal pain and headaches. Biliary colic may occur due to dying worms. While no harms have been found with use during pregnancy, triclabendazole has not been well studied in this population. Triclabendazole is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. It is not commercially available in the United States.