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What are the risks, side effects, and toxic doses of ivermectin in humans, and how do they differ by age and weight?
Executive summary
Approved human ivermectin dosing for parasitic infections is weight‑based: commonly 150–200 micrograms per kilogram as a single oral dose for adults and children who weigh ≥15 kg, and many authorities caution against use in children <15 kg because studies are limited [1] [2] [3]. Serious toxicity in humans is uncommon at therapeutic doses but has been reported with overdose or use of veterinary formulations; animal LD50s and extrapolations provide a very wide putative human LD50 range (≈2.0–43.2 mg/kg) that far exceeds approved single doses (≈0.15–0.20 mg/kg) but is not a substitute for human clinical data [4] [3] [5].
1. How ivermectin is dosed in people — weight matters
Ivermectin tablets approved for human parasitic infections are prescribed by body weight: typical regimens use 150–200 micrograms per kilogram (0.15–0.20 mg/kg) as a single oral dose, with tablet sizes commonly 3, 6, or 12 mg and dose rounding by weight bands in clinical practice [1] [3] [2]. Regulatory materials and clinical guides repeatedly state that dosing decisions for adults and children ≥15 kg should be weight‑based and determined by a clinician [1] [2].
2. Age limits and the 15‑kg contraindication — what reporting shows
Multiple clinical summaries and systematic reviews note a longstanding contraindication or caution for oral ivermectin in children under 15 kg because appropriate studies are limited; that restriction has practical consequences because alternatives for scabies or strongyloidiasis may be less effective or carry other risks [1] [6] [2]. Systematic review authors have explicitly asked whether it is time to reconsider the contraindication because it limits access to an effective drug, but the available pediatric safety data are described as incomplete [6].
3. Typical side effects at approved doses
Sources list common adverse effects seen with appropriate human use—gastrointestinal upset (nausea, vomiting, diarrhea), dizziness, fatigue, rash, and transient neurologic complaints such as drowsiness or headache—and note the need to monitor for interactions with other drugs [7] [8]. Dermatologic reactions and lymph node swelling are also reported in lay and clinical summaries as possible side effects [8] [7].
4. Overdose and toxicity — real‑world drivers of harm
Public‑facing agencies and clinical resources warn that serious toxicity has occurred when people ingest veterinary formulations (highly concentrated animal products) or take much larger doses than prescribed; poison control centers reported increased ivermectin toxicity cases during the COVID‑19 pandemic tied to off‑label or veterinary use [5] [9] [8]. Reported symptoms from large or inappropriate exposures include severe neurologic effects (confusion, decreased consciousness), gastrointestinal symptoms, and other systemic reactions [5] [7].
5. What animal LD50 data say — useful context, not a human threshold
Preclinical LD50 values vary by species (e.g., mice ~25 mg/kg oral, dogs ~80 mg/kg), and literature‑based human‑equivalent extrapolations produce a wide estimated LD50 range (~2.02–43.24 mg/kg). Those numbers are orders of magnitude higher than approved single human doses (~0.15–0.20 mg/kg) but are only contextual: animal LD50s do not define safe human dosing and do not capture idiosyncratic reactions or interactions with human transporters like P‑glycoprotein [4] [10].
6. Pharmacology and mechanisms relevant to toxicity
Ivermectin's antiparasitic effect reflects binding to invertebrate glutamate‑gated chloride channels, but at high concentrations it can interact with mammalian receptors (GABA, glycine, others), and the blood‑brain barrier’s P‑glycoprotein efflux pump protects the CNS; individuals with impaired P‑gp function or very high systemic levels (e.g., overdose) may be at risk for CNS toxicity [10] [7]. Genetic and drug‑interaction variability in CYP3A and P‑gp could alter ivermectin metabolism or CNS exposure, which has been proposed as an explanation for interindividual differences [7] [10].
7. Differences by age and weight — what sources explicitly state
Clinical guidelines and reviews emphasize weight‑based dosing for those ≥15 kg and say appropriate pediatric studies are lacking for children <15 kg; geriatric‑specific problems were not demonstrated in trials but older adults may require caution because of comorbid liver, kidney, or cardiac disease affecting drug handling [1] [3] [6]. In neonates and very young infants, developmental differences in blood‑brain barrier and P‑gp expression are discussed in reviews as biologically relevant, but direct human toxicity thresholds for infants are not established in the cited literature [6] [10].
8. Policy and public‑health disagreements to note
Regulators including the FDA and major health agencies do not authorize ivermectin for COVID‑19 and warn against veterinary product use; some regions and expert groups debated pediatric weight cutoffs and are re‑examining the evidence, illustrating a tension between access for parasitic diseases and safety caution where pediatric data are sparse [9] [6] [11]. The scientific literature and public coverage converge on the same practical advice: follow weight‑based human prescriptions, avoid veterinary products, and consult clinicians for special populations [5] [8].
Limitations: available sources do not give a definitive clinical human LD50 or precise age‑stratified toxic dose thresholds in humans; much toxicity guidance relies on case reports, animal data, pharmacology, and regulatory cautions rather than randomized human overdose trials [4] [6].