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Fact check: What are the active ingredients in horse paste and how do they interact with skin cancer cells?
Executive Summary
Horse “paste” formulations commonly discussed in lay contexts contain different active molecules depending on product: veterinary ivermectin in deworming pastes, plant-derived cytotoxics like betulinic acid (BA) used experimentally on equine melanomas, and solasodine rhamnosyl glycosides (SRGs) found in topical agents such as Curaderm BEC5. Laboratory and animal-model studies report that BA, SRGs and ivermectin each can trigger cancer-cell death pathways or modulate tumor biology, but the evidence is heterogeneous, dated 2010–2022, and does not support off-label self-treatment on humans without clinical trials [1] [2] [3] [4] [5].
1. Why people mention “horse paste” — the ingredients behind the term
The colloquial “horse paste” most often refers to veterinary anthelmintic pastes whose primary active ingredient is ivermectin, a macrocyclic lactone originally developed as an antiparasitic. Research into ivermectin’s anticancer potential reported cytotoxic activity against melanoma cells and synergy with anti-BRAF therapies, and mechanistic studies showed ivermectin can reshape tumor immune microenvironments and interact with inflammatory mediators like gasdermin D, affecting metastasis processes in mouse models [3] [4]. Separate veterinary oncology work identified betulinic acid and derivatives as topical agents for equine melanocytic tumors, while independent dermatologic literature describes SRGs (from plants such as eggplant or devil’s apple) used as topical antitumor agents under trade names like Curaderm BEC5 [1] [2] [6].
2. What the lab and animal data actually show about tumor-killing effects
Cell-culture and animal-model studies document apoptosis induction and tumor regression signals for these compounds. Betulinic acid and its derivative NVX-207 induced apoptotic markers and reduced equine melanoma volumes in vivo, suggesting topical application could be feasible in horses, though authors called for more rigorous trials [1]. SRGs applied topically and intralesionally reported rapid removal of large skin cancers with favourable cosmetic outcomes in the studies cited, framing SRGs as cytotoxic agents that mimic or complement antimitotic therapies [2] [6]. Ivermectin studies found antimelanoma activity and immune-modulatory effects that limited metastasis in mouse and in vitro systems [3] [4].
3. Where the evidence is strongest — and where it falls short
The strongest support is preclinical: cell death in vitro and tumor-size reductions in animal or veterinary case series for BA, SRGs and ivermectin [1] [2] [3] [4]. What’s missing is contemporary, large-scale randomized clinical data demonstrating safe, effective topical or systemic use for human skin cancers. The Curaderm/SRG literature dates to 2012 and contains product-focused outcomes that may reflect smaller series or industry-associated studies [2] [6]. The BA equine study is 2021 but limited to 18 horses and called for more research; ivermectin antitumor work spans mechanistic lab work to mouse metastasis models without human oncology trials [1] [3] [4].
4. Safety, dosage uncertainty, and real-world implications
None of the supplied analyses provide definitive human safety or dosing guidance for these agents when repurposed from veterinary to human use. Topical betulinic acid and SRG treatments were studied within controlled veterinary or dermatologic contexts, not as consumer-grade “pastes.” Ivermectin’s antiparasitic formulations differ by concentration and excipient; preclinical anticancer doses or delivery methods may not translate safely to human skin cancer therapy without trials. The literature emphasizes biological plausibility and early efficacy signals but does not validate home-use substitution of regulated cancer treatment [1] [2] [3] [4] [5].
5. Competing narratives and potential agendas to watch
Studies of Curaderm/SRGs and topical BA often present rapid, cosmetically favourable outcomes and may be promoted as alternatives to surgery; these narratives can reflect commercial or treatment-innovation agendas from product developers or small clinical series [2] [6]. Ivermectin literature spans antiparasitic repurposing and broader drug-repurposing enthusiasm; that field attracts advocacy beyond established oncology, creating a risk of overstating preclinical promise [3] [4]. Importantly, independent peer-reviewed replication and human clinical trials are the standard to temper promotional claims, and those are lacking in the supplied dataset [1].
6. Bottom line for clinicians and patients considering these reports
The scientific record in these sources shows biological activity of betulinic acid, SRGs and ivermectin against melanoma and nonmelanoma skin cancer cells in laboratory and veterinary contexts, with some promising in vivo results in horses and mice. However, the evidence remains preclinical or limited clinical case series, dated from 2010–2022, and cannot justify off-label consumer use of veterinary pastes for human skin cancer. Formal human trials, standardized formulations, and safety data are required before these agents should be recommended beyond regulated clinical research settings [1] [2] [3] [4] [5].