evidence: Toxic Effects from Ivermectin Use Associated with Prevention and Treatment of Covid-19 [...] Of the 21 persons who called in August, 11 were men, and most were older than 60 years of age (median age, 64; range, 20 to 81). Approximately half (11 persons) were reported to have used ivermectin to prevent Covid-19, and the remaining persons had been using the drug to treat Covid-19 symptoms. Three persons had received prescriptions from physicians or veterinarians, and 17 had purchased veterinary formulations; the source of ivermectin for the remaining person was not confirmed. Symptoms had developed in most persons within 2 hours after a large, single, first-time dose. In 6 persons, symptoms had developed gradually after several days to weeks of repeated doses taken every other day or twice weekly. One person had also been taking vitamin D to treat or prevent Covid-19. Reported doses ingested by the persons who had been using veterinary products ranged from 6.8 mg to 125 mg of 1.87% paste and 20 to 50 mg of the 1% solution. The dose of the human-use tablets was 21 mg per dose twice weekly for prevention. [...] These cases illustrate the potential toxic effects of ivermectin, including severe episodes of confusion, ataxia, seizures, and hypotension, and the increasing frequency of inappropriate use. There is insufficient evidence to support the use of ivermectin to treat or prevent Covid-19,3 and improper use, as well as the possible occurrence of medication interactions,5 may result in serious side effects requiring hospitalization.