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Fact check: Can ivermectin in horse paste be absorbed through human skin and cause systemic effects?
Executive Summary
Ivermectin in veterinary formulations is designed for oral or pour-on use in animals and is not readily absorbed through intact human skin in its raw paste form under typical exposures, but specialized formulations and certain conditions can permit transdermal delivery, so skin contact with horse paste is unlikely to cause systemic effects in most cases while not being risk-free [1] [2]. The scientific literature shows low transdermal bioavailability from pour-on veterinary products and limited systemic absorption from topical human creams, yet recent experimental nanoformulations demonstrate that engineered transdermal vehicles can deliver ivermectin through human skin, indicating context matters for risk assessment [3] [4] [2].
1. Why people worry: medieval practice meets modern misuse and why the question matters
Public concern arises because people have repurposed veterinary ivermectin products for human use, prompting questions about whether handling or applying horse paste could produce systemic poisoning. Clinical reviews outline ivermectin’s pharmacokinetics—rapid absorption orally but limited systemic uptake from topical human creams—while case reports of misuse highlight harms from ingesting veterinary formulations rather than dermal exposure [1] [4]. Studies of pour-on veterinary administrations in horses show notably lower plasma availability compared with oral dosing, which supports the view that protocols designed for animals do not reliably create high human systemic levels via skin contact [3] [5]. The misuse reporting also underscores that the main documented harms come from ingestion and dosing errors, not from normal dermal contact, though the misuse context often conflates routes and formulations [6].
2. The laboratory reality: topical human creams versus veterinary pour-on and paste
Controlled pharmacokinetic studies of ivermectin 1% human creams demonstrate minimal transdermal absorption and clinical pharmacokinetic profiles consistent with limited systemic exposure after dermal application, which supports safety for approved topical indications when used as directed [4]. In contrast, veterinary "pour-on" products tested in horses show poor plasma bioavailability compared to oral administration, meaning the formulation and route drastically alter how much drug reaches systemic circulation [3] [5]. Experimental ex vivo and novel nano-delivery work shows that ivermectin can be intentionally formulated to penetrate human skin effectively, which proves transdermal delivery is scientifically possible but contingent on the vehicle and formulation strategy rather than inherent to ivermectin paste itself [2] [7]. Therefore, formulation and application method are decisive for transdermal uptake.
3. Conflicting evidence and the technical caveat that changes risk assessment
Recent nanoformulation experiments demonstrate successful transdermal delivery of ivermectin through human skin models without high cytotoxicity, which contradicts blanket claims that ivermectin cannot cross intact skin [2]. These are laboratory demonstrations using engineered carriers and ex vivo skin setups; they do not equate to typical household exposures to veterinary paste but they do change the risk calculus by proving the barrier is surmountable if the formulation is optimized [2]. Meanwhile, veterinary pharmacology studies showing low systemic levels after pour-on use in animals indicate that common commercial animal preparations and routes were not designed to maximize systemic human absorption, and thus real-world dermal exposures to horse paste are unlikely to mimic the conditions of engineered transdermal systems [3] [5]. Both sets of findings are accurate within their experimental contexts.
4. Real-world harms: ingestion dominates, but dermal scenarios deserve nuance
Clinical and public-health reports of harm from veterinary ivermectin misuse predominantly document effects after oral ingestion or deliberate high-dose use, not from incidental skin contact with equine paste [6] [1]. That evidence supports guidance against ingesting veterinary products and argues for clinician counseling focused on ingestion risks. However, the existence of transdermal delivery research and variable veterinary formulations means occupational or prolonged direct-skin exposures—especially combined with solvents, damaged skin, or occlusive dressings—could theoretically increase systemic uptake beyond typical incidental contact [2] [3]. Thus, common household handling of a tube of horse paste poses low systemic risk from skin alone, but documented experimental penetration and real-world misuse justify caution and medical consultation if unusual exposure occurs [2] [6].
5. Bottom line for clinicians, workers, and the public — practical guidance
Treat veterinary ivermectin paste as unsafe for human ingestion and avoid intentional application to skin; routine incidental dermal contact is unlikely to cause systemic toxicity, but prolonged contact, application over broken skin, use with penetration enhancers, or exposure to specially formulated transdermal carriers could change that assessment [1] [2] [3]. Healthcare advice should emphasize that the primary documented harms come from oral misuse of veterinary products, while also recognizing that emerging research proves transdermal delivery is possible under engineered conditions—this underscores the importance of formulation context when evaluating exposure risk [6] [2]. If significant exposure occurs, especially with symptoms, clinicians should evaluate for systemic effects and manage according to toxicology guidance rather than assuming dermal contact was harmless [1].