What is the appropriate human dose of ivermectin 1.87% (18.7 mg/mL) oral solution?

Human Dose for Ivermectin 1.87% Oral Solution

For human use, ivermectin 1.87% (18.7 mg/mL) oral solution should be dosed at 200 mcg/kg body weight as a single oral dose, repeated in 2 weeks for scabies, or 150 mcg/kg for onchocerciasis. 1, 2

Dose Calculation for 1.87% Solution

Since the concentration is 18.7 mg/mL (1.87%), you need to calculate the volume based on patient weight:

• For scabies (200 mcg/kg):

  • Example: 70 kg patient needs 14 mg total dose
  • Volume = 14 mg ÷ 18.7 mg/mL = 0.75 mL

• For onchocerciasis (150 mcg/kg):

  • Example: 70 kg patient needs 10.5 mg total dose
  • Volume = 10.5 mg ÷ 18.7 mg/mL = 0.56 mL

Administration Guidelines

Ivermectin should be taken with food because bioavailability is increased by approximately 2.5-fold, thereby increasing penetration into the epidermis 1, 2. This is critical for efficacy in parasitic skin conditions. The UK guidelines specifically note to "take without food, but with water" for certain helminthic infections, but emphasize that "high fat food increases bioavailability by 2.5 times" 3, which is beneficial for scabies treatment.

Dosing by Indication

Scabies

• Standard dose: 200 mcg/kg orally, repeated in 2 weeks 1
• Why repeat dosing? Ivermectin has limited ovicidal activity and may not prevent recurrences from eggs present at treatment; therefore, a second dose 14 days after the first is essential 1
• Recent evidence supports higher dosing: A 2017 study found that 400 mcg/kg as a single dose achieved complete resolution in all patients, whereas 200 mcg/kg required 2-3 doses 4. However, guidelines have not yet incorporated this higher dose.

Crusted (Norwegian) Scabies

• Intensive regimen: 200 mcg/kg on days 1,2,8,9, and 15, combined with daily topical permethrin 1
• Additional doses on days 22 and 29 may be required for severe cases 1

Onchocerciasis

• Standard dose: 150 mcg/kg as a single oral dose 2
• Retreatment intervals: 12 months in mass campaigns, or as short as 3 months for individual patients 2

Special Populations

Children

• Weight <15 kg: Use is off-label but appears safe. A multicenter study of 170 children (4-14.5 kg) showed only 4% mild adverse events with mean dose of 223 mcg/kg 5
• Efficacy improved when dose exceeded 200 mcg/kg and interval between doses was <10 days 5
• Children <10 years: Should not receive lindane as alternative 1

Pregnancy and Lactation

• Pregnancy: Classified as "human data suggest low risk" - no teratogenicity or toxicity observed in human pregnancy experience 1
• Lactation: Very low levels excreted in breastmilk, probably compatible with breastfeeding 1, 3

Renal Impairment

• No dosing adjustments required for renal impairment 1

Hepatic Impairment

• Caution advised: Safety of multiple doses in severe liver disease is not known 1
• Ivermectin is primarily metabolized by CYP3A4 2

Critical Drug Interactions

Azithromycin can significantly increase serum ivermectin concentration 3. Exercise caution with concomitant use.

CYP3A4 inducers (anticonvulsants, steroids) may reduce ivermectin levels 3.

Common Pitfalls to Avoid

1. Single-dose failure: The most common predictor of treatment failure is absence of a second dose (P < 0.001) 6. A 2023 study showed only 58% success with single dose vs. 98% with two doses one week apart.

2. Fasting administration: Taking ivermectin on an empty stomach reduces bioavailability by 60% 2. Always administer with food for scabies treatment.

3. Premature retreatment: Pruritus may persist for 2+ weeks after successful treatment due to allergic dermatitis, not treatment failure 1. Only retreat if live mites are observed or symptoms persist beyond 2 weeks.

4. Eczematous eruptions: New cutaneous lesions may appear within 72 hours post-treatment in up to 48% of patients, representing subacute eczema rather than treatment failure 4. Treat with topical corticosteroids.

5. Elderly/debilitated patients: One study showed increased mortality in this population, though not confirmed in subsequent reports 1. Use with appropriate caution.

Monitoring

• No routine laboratory monitoring required for standard dosing
• Follow-up examination at 1-2 weeks to assess for persistent infestation 1
• For crusted scabies, close monitoring for treatment response and potential need for additional doses

Contraindications

• Loa loa co-infection: Exclude before use due to risk of severe reactions 3
• Onchocerciasis co-infection: Use with care; prednisolone usually given alongside when microfilaraemic 3