What are the adverse effects of using veterinarian ivermectin in humans?

Adverse Effects of Veterinary Ivermectin in Humans

Veterinary ivermectin formulations pose serious toxicity risks in humans, with documented cases of severe neurotoxicity, gastrointestinal distress, and even death following accidental or intentional exposure—particularly when administered intravenously or at high doses intended for animals.

Critical Distinction: Veterinary vs. Human Formulations

Veterinary ivermectin products are significantly more concentrated than human formulations and are never intended for human use. 1 The FDA drug label explicitly warns that accidental intoxication with veterinary formulations can cause severe adverse effects through ingestion, inhalation, injection, or skin exposure. 1

Documented Adverse Effects from Veterinary Formulations

Neurological Toxicity (Most Serious)

• Severe neurotoxicity is the predominant concern, with altered mental status occurring more frequently in patients taking veterinary products compared to prescription tablets 2
• Documented effects include:
  • Seizures 1
  • Ataxia (loss of coordination) 1, 2
  • Altered mental status 2
  • Tremors 1
  • Ptosis (drooping eyelids) 1
  • Mydriasis (pupil dilation) 1
  • Paresthesia (abnormal sensations) 1

A case report documented unprecedented severe neurotoxicity requiring ICU admission after intravenous administration of veterinary ivermectin, with toxic serum concentrations of 187.74 ng/mL. 3

Gastrointestinal Effects

• Nausea and vomiting 1, 2
• Diarrhea 1
• Abdominal pain 1

Cardiovascular and Respiratory Effects

• Bradypnea (slow breathing) 1
• Dyspnea (difficulty breathing) 1
• Clinically significant hypotension 1

Dermatological Reactions

• Rash 1
• Edema 1
• Urticaria (hives) 1
• Contact dermatitis 1

Other Systemic Effects

• Headache 1
• Dizziness 1
• Asthenia (weakness) 1
• Musculoskeletal complaints 2

Dose-Dependent Toxicity Patterns

Patients taking veterinary formulations ingested significantly higher doses than those using prescription tablets, leading to more severe toxicity. 2

Acute High-Dose Exposure

• Veterinary product users took large single doses or large daily doses over several days 2
• Rapid onset of severe neurotoxicity 2
• Higher rates of altered mental status 2

Chronic Lower-Dose Exposure

• Patients taking typical therapeutic doses chronically (median 3.8 weeks, daily dose 13.5 mg) developed milder symptoms 2
• Toxicity still occurred despite "therapeutic" dosing when continued for weeks rather than days 2

Lethality Data

Animal studies demonstrate significant lethality at high doses: single oral doses of 25-50 mg/kg in mice and rats, and 40-50 mg/kg in rats caused death. 1 Dogs showed no significant lethality up to 10 mg/kg. 1

Human fatalities have been reported: In a case series of 37 ivermectin toxicity cases during COVID-19, one patient died. 2

Special Risk Factors

P-glycoprotein Considerations

• Ivermectin is a substrate for P-glycoprotein, which normally limits neurological toxicity 4
• Genetic polymorphisms in P-glycoprotein or co-administration of P-glycoprotein inhibitors may increase neurotoxicity 4

Liver Disease

• Patients with severe liver disease may be at increased risk for toxicity 5

Co-infection with Loa loa

In patients with high Loa loa microfilarial loads (>8,000 microfilaria/ml), ivermectin can cause serious neurological adverse events that may lead to death, with risk increasing dramatically above 30,000 microfilaria/ml. 6

Demographics of Toxicity Cases

Ivermectin toxicity during COVID-19 predominantly affected male patients over 60 years old who ingested higher than recommended doses. 2 The median age was 64 years, with most patients requiring hospitalization (21) or emergency department treatment (13). 2

Emergency Management

The FDA drug label recommends the following for veterinary ivermectin poisoning: 1

• Supportive therapy with parenteral fluids and electrolytes
• Respiratory support (oxygen and mechanical ventilation if necessary)
• Pressor agents for clinically significant hypotension
• Induction of emesis and/or gastric lavage as soon as possible
• Purgatives and routine anti-poison measures to prevent absorption

Context: COVID-19 Misuse

The WHO strongly recommends against ivermectin for COVID-19 outside clinical trials, noting it diverts resources from proven therapies. 6, 5 Moderate certainty evidence suggests ivermectin may increase the risk of serious adverse events in hospitalized COVID-19 patients (RR 3.10,95% CI 0.54-17.89). 5

Critical Clinical Pitfall

The most dangerous scenario is intravenous administration of veterinary ivermectin, which is formulated only for animal subcutaneous injection and has caused severe neurotoxicity requiring ICU care when given IV to humans. 3 This route of administration should never occur in humans under any circumstances.