Is it okay to repeat ivermectin (antiparasitic medication) in a patient with a history of scabies who previously took ivermectin a few months ago and reports that permethrin (pyrethroid insecticide) is ineffective?

Yes, it is safe and appropriate to repeat ivermectin now for persistent scabies

Given that permethrin has failed and several months have passed since the last ivermectin dose, repeating ivermectin is both safe and indicated, with the critical caveat that a two-dose regimen (repeated at 2 weeks) must be used to address ivermectin's limited ovicidal activity. 1

Why Repeating Ivermectin is Appropriate

Standard Dosing and Safety Profile

• Ivermectin 200 μg/kg orally is a CDC-recommended first-line treatment for scabies, equivalent in status to permethrin 1, 2
• The medication has an established safety profile with no specific contraindications to repeated courses after several months 1
• No dosage adjustments are required for patients with renal impairment, though caution is warranted in severe liver disease 1, 3
• The FDA label confirms that repeated doses at intervals as short as 3 months are acceptable for onchocerciasis, supporting safety of re-dosing 4

Critical Treatment Requirements

• You must prescribe TWO doses of ivermectin, 2 weeks apart, not a single dose 1, 2
• Ivermectin has limited ovicidal activity and does not kill eggs effectively, making the second dose essential to eliminate mites that hatch after initial treatment 1
• Recent evidence from 2025 shows that treatment failure is associated with not repeating the ivermectin dose 5
• Instruct the patient to take ivermectin with food to increase bioavailability and epidermal penetration 1, 2

Addressing Treatment Failure

Why Prior Treatment May Have Failed

• Permethrin resistance is documented in some geographic areas 1
• Inadequate application of topical permethrin (not covering all body areas from neck down) 1, 2
• Failure to treat all household contacts and close contacts simultaneously 2, 6
• Inadequate environmental decontamination of bedding and clothing 1, 2
• If only a single dose of ivermectin was given previously, eggs would have survived and re-established infestation 1

Essential Concurrent Measures

• Treat all household members, sexual partners, and close contacts within the past month simultaneously, even if asymptomatic 2, 6
• Decontaminate all bedding, clothing, and towels by machine washing/drying on hot cycle or dry cleaning 1, 2, 6
• Items that cannot be washed should be removed from body contact for at least 72 hours 1, 2
• Recent data suggests storing clothes in plastic bags for 4 days and vacuum cleaning furniture/car seats improves outcomes 5

Follow-Up Expectations

Normal Post-Treatment Course

• Pruritus and rash may persist for up to 2 weeks after successful treatment due to ongoing hypersensitivity reaction to dead mites 1, 2, 6
• This does NOT indicate treatment failure and should not prompt immediate retreatment 1, 2

When to Consider True Treatment Failure

• Symptoms persisting beyond 2 weeks warrant re-evaluation 1, 2, 6
• Look for live mites on skin scraping or new burrows appearing after the 2-week mark 2, 6
• Consider crusted (Norwegian) scabies if patient is immunocompromised, which requires more aggressive combination therapy 1, 2

Special Considerations

Contraindications to Verify

• Ivermectin should not be used in children weighing <15 kg 6, 4
• Use caution in pregnancy (though human data suggest low risk) and lactation—permethrin is preferred in these populations 1, 6
• Avoid in patients with severe liver disease due to uncertain safety with multiple doses 1, 3

If This Treatment Course Fails

• Consider crusted scabies, especially if patient has HIV, is on immunosuppressants, or is otherwise immunocompromised 1, 2
• Crusted scabies requires combination therapy: topical permethrin daily for 7 days then twice weekly PLUS oral ivermectin on days 1,2,8,9, and 15 1, 2, 6
• Verify that all contacts were treated and environmental decontamination was thorough 2, 5