[andrew read]

Benzimidazole carbamates such as fenbendazole are often recommended to treat amphibian helminth infestations, however there is not much information on the adverse side effects caused by these chemicals.
I have myself noted that at a concentration of only 20 parts of fenbendazole per billion (thousand million) of water, long term exposure causes inappetance in amphibians, and at a concentration of only 200 parts per billion long term exposure results in death. I have seen similar side effects with mebendazole, albendazole and thiophanate.
Many different benzimidazoles have been developed over the years to treat human and animal diseases and also as pesticides for agriculture and horticulture. Such chemicals include the anthelmintic benzimidazole carbamates such as fenbendazole, mebendazole, albendazole, flubendazole, oxfendazole and oxibendazole, the anthelmintic probenzimidazoles such as febantel, netobimin and thiophanate, and the related anthelmintic benzimidazoles such as tiabendazole and triclabendazole. In agriculture related compounds such as benomyl are used as fungicides.
It can be expected that many if not all of these chemicals will have similar side effects on amphibians. Because of their widespread use residues in soil and in small ponds might reach the above concentration values in some places and have adverse effects on amphibian populations; some care over their use might therefore be a good idea I think.
In contrast the antifungal imidazoles such as ketoconazole and clotrimazole do not appear to have these adverse side effects, nor do the antifungal triazoles such as itraconazole, nor the antiprotozoal nitroimidazoles such as metronidazole and dimetridazole.