What are the contraindications for ivermectin (including hypersensitivity, weight <15 kg, pregnancy, lactation, severe hepatic impairment, high Loa loa microfilariae load, and concurrent strong P‑glycoprotein inhibitors)?

Contraindications for Ivermectin

Ivermectin is contraindicated in patients with known hypersensitivity to the drug, and should not be used during pregnancy due to established teratogenicity in animal studies. 1

Absolute Contraindications

Hypersensitivity

• Any documented allergic reaction to ivermectin is an absolute contraindication 1

Pregnancy

• Ivermectin should not be used during pregnancy as safety has not been established 1
• Animal studies demonstrate teratogenicity at 0.2-8.1 times the maximum human dose, characterized by cleft palate and clubbed forepaws 1
• While systematic review data shows insufficient evidence to definitively conclude on safety, the FDA label explicitly states it should not be used in pregnancy 1, 2
• The drug is classified as Pregnancy Category C 1

High-Risk Situations Requiring Extreme Caution

High Loa loa Microfilaremia

This represents the most critical safety concern with ivermectin use:

• Risk of fatal encephalopathy exists in patients heavily co-infected with Loa loa 1
• The 8,000 mf/mL threshold is widely recognized as the point requiring altered treatment strategies 3
• Recent evidence suggests SAE probability exceeds 1% when MFD >20,000 mf/mL and >2.5% when MFD >30,000 mf/mL 4
• Males aged 21-50 and females aged 31-40 are at higher risk and should use lower thresholds (12,000-19,000 mf/mL depending on age/sex) 4
• Pretreatment assessment for loiasis is mandatory in patients with significant exposure to West or Central African endemic areas 1

Clinical manifestations of Loa loa-associated encephalopathy include: pain (neck/back), red eye, conjunctival hemorrhage, dyspnea, incontinence, difficulty walking, confusion, lethargy, stupor, seizures, or coma 1

Pediatric Weight Restrictions

• Safety and effectiveness not established in children weighing <15 kg 1
• This is the FDA-labeled contraindication, though observational data from 170 children (4-14.5 kg) showed only 4% mild adverse events with no serious events 5
• The FDA restriction remains the standard of care despite emerging evidence

Lactation

• Ivermectin is excreted in human milk in low concentrations 1
• Treatment should only occur when risk of delayed maternal treatment outweighs possible risk to the newborn 1

Relative Contraindications and Special Populations

Severe Hepatic Impairment

• While not explicitly contraindicated in the FDA label, one case report documents ivermectin-induced severe hepatitis with confluent necrosis 6
• Exercise caution in elderly patients with decreased hepatic function 1

P-glycoprotein Inhibitor Co-administration

• Concurrent use of strong P-glycoprotein inhibitors may increase neurotoxicity risk 7, 8
• P-glycoprotein normally limits ivermectin brain penetration; inhibition can elevate CNS concentrations 7
• Common P-glycoprotein inhibitors include certain antifungals, calcium channel blockers, and immunosuppressants

Drug Interactions

• Post-marketing reports show increased INR when co-administered with warfarin 1
• Monitor coagulation parameters closely if concurrent use is necessary

Immunocompromised Patients

• Multiple treatment courses may be required for strongyloidiasis 1
• Cure may not be achievable; suppressive monthly therapy may be needed 1

Critical Clinical Pitfalls

1. Never assume absence of Loa loa in patients from endemic areas - always assess microfilarial load before treatment 1
2. Age and sex matter for Loa loa risk stratification - males 21-50 and females 31-40 require lower MFD thresholds 4
3. The 30,000 mf/mL threshold may be too high - significant SAE risk exists at 20,000 mf/mL 4
4. Patients with hyperreactive onchodermatitis (sowda) are at higher risk for severe reactions including edema 1