What are the differences in toxicity between veterinary and human formulations of ivermectin?
Executive summary
The clinical literature and poison‑center reporting show a consistent pattern: veterinary ivermectin exposures more often involve large, untested doses and formulations (pastes, solutions) that produce rapid, severe neurotoxicity, while human‑prescription ivermectin causes toxicity chiefly when taken at higher‑than‑recommended doses or for prolonged periods [1] [2]. Veterinary products also contain excipients and secondary actives not evaluated for human safety, increasing unpredictable risks [2] [3].
1. How formulations differ in concentration and presentation
Veterinary ivermectin is sold in high‑concentration pastes, solutions and injectables intended for large animals, whereas human formulations typically come as prescription tablets or topical creams at doses approved for parasitic diseases; these distinct presentations make accidental or intentional overdosing with veterinary products more likely [2] [4].
2. Clinical patterns: acute neurotoxicity vs. chronic lower‑grade effects
A clinical toxicology series identified 37 cases of ivermectin toxicity during the COVID period and reported that patients who ingested veterinary formulations tended to take large single doses or repeated large daily doses and developed rapid onset neurotoxicity, while patients exposed to human formulations more often had milder symptoms with prolonged, lower‑dose exposures [1] [5] [2].
3. Severity, demographics and outcomes observed in reports
In the reviewed cohort the median age was 64, most patients were male, the majority required emergency care or hospitalization, and one death was reported; veterinary‑product exposures disproportionately produced severe neurologic symptoms and quicker decompensation [1]. Public health call data also documented a marked rise in veterinary‑product exposures and related harms during the pandemic [3].
4. The hidden role of excipients and untested additives
Veterinary ivermectin preparations can include fillers, solvents, or secondary actives such as clorsulon that are not tested for human safety; these untested excipients may contribute to toxicity or alter absorption, creating risks beyond what the ivermectin molecule alone would present [2] [3]. Poison‑control guidance explicitly warns that veterinary products “have not been tested in humans” and can deliver excessive or unpredictable doses [3].
5. Availability, pricing and why people used veterinary products
A shortage of human‑prescription ivermectin and widespread misinformation led some individuals to seek veterinary products, driving price spikes and increased off‑label consumption of animal formulations — a market pressure that correlated with more poison‑center calls and documented exposures [6] [3]. Marketplace ease of purchase for veterinary formulations thus amplified the public‑health impact seen in clinical series [6].
6. Practical difference in toxicity risk: dose, formulation and unknowns
The fundamental difference in toxicity risk is pragmatic: veterinary formulations are more likely to deliver much higher ivermectin doses per administration and include non‑human excipients, producing a higher probability of acute neurotoxicity; human formulations, when used within approved dosing, are not expected to cause severe toxicity, but overdose or prolonged misuse can still produce adverse neurologic and gastrointestinal effects [1] [2] [3]. Reports reviewed do not claim intrinsic molecular differences between “veterinary” and “human” ivermectin beyond concentration and formulation, but they emphasize that the real‑world toxicity gap stems from dosing, formulation, and untested additives [1] [2].
7. Limitations, alternative views and public‑health messaging
Available reporting is based on case series, poison‑center calls and observational data that cannot establish incidence or all causal mechanisms; while toxicologists and poison centers warn against using veterinary products in humans because of unknown excipients and overdose risk, proponents of ivermectin use for COVID‑19 focused on purported antiviral benefits rather than safety data — a perspective not supported by the toxicity patterns documented in poison‑center and clinical toxicology reports [3] [1]. Regulatory and clinical sources therefore urged against substituting veterinary products for human‑prescription medication because the safety profile in humans has not been established for those formulations [3] [4].