Mechanism of action
Ivermectin binds selectively and with high affinity to glutamate-gated chloride ion channels present in invertebrate nerve and muscle cells. This binding increases chloride ion permeability of the cell membrane, causing hyperpolarization and resulting in paralysis and death of the parasite.
The drug is selective for invertebrates because these glutamate-gated chloride channels do not exist in mammals. Ivermectin also interacts with other ligand-gated chloride channels, such as those gated by gamma-aminobutyric acid (GABA), but in mammals these channels are protected by the blood-brain barrier. P-glycoprotein pumps at the blood-brain barrier normally exclude ivermectin from the central nervous system.
Ivermectin is microfilaricidal (kills larval forms) for many nematodes but has minimal effect on adult worms of Onchocerca volvulus. It is, however, adulticidal for Strongyloides stercoralis. The drug also disrupts parasite reproduction and may have anti-inflammatory effects that contribute to therapeutic efficacy in certain conditions.
- Ivermectin binds glutamate-gated chloride channels in parasites, causing hyperpolarization and paralysis.
- Fatal encephalopathy can occur in Loa loa co-infection due to massive microfilarial death; screen before treatment in endemic areas.
- Drug of choice for onchocerciasis (river blindness) and strongyloidiasis, including disseminated disease.
- Mazzotti reaction (fever, pruritus, hypotension) results from dying Onchocerca microfilariae; treat symptomatically.
- P-glycoprotein at the blood-brain barrier protects mammals from CNS toxicity; inhibitors increase neurotoxicity risk.
- Microfilaricidal for Onchocerca but not adulticidal; repeated annual dosing required.
- Useful for crusted (Norwegian) scabies, often combined with topical permethrin.
- Long half-life (12-36 hours) allows single-dose or infrequent dosing, improving compliance in endemic regions.
Indications
Ivermectin is FDA-approved for the treatment of:
- Onchocerciasis (river blindness) caused by Onchocerca volvulus: ivermectin is the drug of choice, killing microfilariae and preventing transmission by blackflies
- Strongyloidiasis caused by Strongyloides stercoralis: effective for both intestinal and disseminated disease
- Scabies caused by Sarcoptes scabiei: particularly useful for crusted (Norwegian) scabies and institutional outbreaks
Off-label and internationally recognized uses include:
- Lymphatic filariasis (in combination with albendazole for mass drug administration programs)
- Cutaneous larva migrans
- Pediculosis (head lice, body lice)
- Rosacea (topical formulation)
- Ascariasis and enterobiasis (less commonly used)
- Scabies prophylaxis in outbreak settings
Ivermectin has been studied for numerous other conditions but lacks high-quality evidence for routine use outside established parasitic indications.
Pharmacokinetics
Absorption: Ivermectin is well absorbed orally, with peak plasma concentrations occurring approximately 4 hours after administration. Absorption is enhanced when taken with a fatty meal.
Distribution: The drug is highly lipophilic and distributes widely into tissues. It does not readily cross the blood-brain barrier in healthy individuals due to P-glycoprotein efflux pumps. Volume of distribution is large due to extensive tissue binding.
Metabolism: Ivermectin undergoes hepatic metabolism primarily via CYP3A4. It has a long half-life of approximately 12-36 hours, allowing for single-dose or infrequent dosing regimens.
Excretion: Elimination occurs primarily via feces (greater than 95%), with minimal renal excretion. The extended half-life results in prolonged therapeutic effects and slow clearance from the body.
Dosing intervals vary by indication and typically follow product labeling, with many parasitic infections treated with single doses or short courses repeated at specific intervals.
Adverse effects
Most adverse effects of ivermectin are mild and transient, though serious reactions can occur in specific populations.
Common adverse effects:
- Dizziness, headache
- Nausea, diarrhea, abdominal pain
- Pruritus and rash
- Fatigue, weakness
- Arthralgia, myalgia
Mazzotti reaction: A syndrome occurring in patients treated for onchocerciasis, caused by the inflammatory response to dying microfilariae. Manifestations include fever, urticaria, pruritus, lymphadenitis, arthralgias, tachycardia, and hypotension. Severity correlates with microfilarial burden. Treatment is supportive, often with antihistamines or corticosteroids.
Severe neurotoxicity: Patients co-infected with Loa loa (endemic in Central Africa) can develop fatal encephalopathy when treated with ivermectin due to massive microfilarial death in the central nervous system. This is the most serious complication and requires screening in endemic areas before treatment. Symptoms include confusion, altered mental status, seizures, coma, and death.
Other serious effects:
- Hepatitis (rare)
- Stevens-Johnson syndrome (very rare)
- Asthma exacerbation
- Orthostatic hypotension
Overdose: May result in CNS depression, ataxia, tremor, and mydriasis. Patients with genetic defects in P-glycoprotein (MDR1 mutations, seen in some Collie dogs) would theoretically be at risk, but this is rare in humans.
Contraindications and warnings
Absolute contraindications:
- Hypersensitivity to ivermectin or any formulation component
Warnings and special populations:
- Loa loa co-infection: Screen patients from endemic areas (Central Africa) before treatment. High microfilarial loads (greater than 30,000 microfilariae/mL) increase risk of fatal encephalopathy. Alternative strategies or careful monitoring are required.
- CNS penetration risk: Patients with conditions that compromise the blood-brain barrier (meningitis, African trypanosomiasis) may be at increased risk for CNS adverse effects.
- P-glycoprotein inhibitors: Concurrent use of drugs that inhibit P-glycoprotein may increase CNS penetration and toxicity.
Pregnancy: Category C. Animal studies have shown teratogenicity at high doses. Use only if benefit clearly outweighs risk. In mass drug administration programs for onchocerciasis, pregnant women are typically excluded.
Lactation: Ivermectin is excreted in breast milk. The risk to nursing infants is considered low, but caution is advised.
Pediatrics: Safety and efficacy not established in children weighing less than 15 kg. Avoid in neonates and young infants due to immature blood-brain barrier.
Hepatic impairment: Use with caution, as ivermectin is hepatically metabolized.
Asthma: May trigger bronchospasm; use cautiously in patients with asthma.
Drug interactions
Ivermectin has relatively few clinically significant drug interactions, but awareness of CYP3A4 and P-glycoprotein interactions is important.
CYP3A4 inhibitors: Drugs such as ketoconazole, itraconazole, erythromycin, and ritonavir may increase ivermectin plasma concentrations by inhibiting metabolism. This could theoretically increase the risk of toxicity.
CYP3A4 inducers: Rifampin, phenytoin, carbamazepine, and St. John's wort may decrease ivermectin levels, potentially reducing efficacy.
P-glycoprotein inhibitors: Agents such as verapamil, cyclosporine, and quinidine may increase CNS penetration of ivermectin by inhibiting efflux pumps at the blood-brain barrier, raising the risk of neurotoxicity.
Warfarin: Isolated reports suggest ivermectin may potentiate anticoagulant effects; monitor INR if co-administered.
Other antiparasitic agents: Ivermectin is often used in combination with albendazole or diethylcarbamazine in mass drug administration programs. Diethylcarbamazine should not be used with ivermectin in patients with onchocerciasis due to increased risk of severe inflammatory reactions.
Alcohol: May theoretically increase CNS penetration; advise avoidance around the time of dosing.
No dose adjustment is typically required for mild to moderate interactions, but clinical judgment and monitoring are warranted in high-risk patients.
Clinical pearls
- Screen for Loa loa before treating patients from Central Africa to prevent fatal encephalopathy. This is the highest-yield safety consideration.
- Ivermectin is microfilaricidal but not adulticidal for Onchocerca volvulus, so repeated dosing (often annually) is required to control onchocerciasis.
- The Mazzotti reaction is a tipoff for onchocerciasis diagnosis and treatment; it reflects therapeutic efficacy but requires symptomatic management.
- For crusted scabies, ivermectin is often combined with topical permethrin due to high mite burden and risk of treatment failure with monotherapy.
- Ivermectin is the drug of choice for Strongyloides hyperinfection syndrome in immunocompromised patients, where untreated infection can be fatal.
- The drug is a cornerstone of mass drug administration programs aimed at eliminating lymphatic filariasis and onchocerciasis in endemic regions.
- P-glycoprotein normally protects the CNS from ivermectin toxicity; conditions or drugs that impair this efflux pump increase neurotoxicity risk.
- Ivermectin has a long half-life, enabling single-dose or widely spaced dosing regimens that improve compliance in resource-limited settings.
- Topical ivermectin is FDA-approved for inflammatory lesions of rosacea, a distinct indication from its systemic antiparasitic use.
- There is no high-quality evidence supporting ivermectin for viral infections; its use should be restricted to proven parasitic indications.
Practice questions
1. A 32-year-old man from Cameroon presents to a rural clinic with severe itching and skin nodules. He has been experiencing gradual vision loss over the past year. Examination reveals subcutaneous nodules and hypopigmented skin patches. Skin snip biopsy demonstrates microfilariae. The physician prescribes ivermectin. Two hours after the first dose, the patient develops fever, pruritus, hypotension, and tachycardia. What is the most likely cause of this reaction?
2. A 45-year-old man from Equatorial Guinea requires treatment for suspected strongyloidiasis. Before prescribing ivermectin, the physician orders screening laboratory tests. Results show 35,000 Loa loa microfilariae per mL of blood. What is the most appropriate next step in management?
3. A 28-year-old immunocompromised patient with HIV/AIDS presents with chronic diarrhea, abdominal pain, and a pruritic linear rash on the trunk. Stool examination reveals larvae of Strongyloides stercoralis. The patient is started on appropriate therapy. Which mechanism best explains the drug's antiparasitic action?
Frequently asked questions
How does ivermectin work?
Ivermectin works by binding to glutamate-gated chloride ion channels in invertebrate nerve and muscle cells, causing hyperpolarization, paralysis, and death of the parasite. These channels do not exist in mammals, making the drug selectively toxic to parasites. In mammals, P-glycoprotein pumps at the blood-brain barrier prevent CNS penetration, protecting against neurotoxicity under normal conditions.
What are the side effects of ivermectin?
Common side effects include dizziness, headache, nausea, diarrhea, pruritus, and fatigue. The Mazzotti reaction (fever, rash, hypotension) occurs when treating onchocerciasis due to dying microfilariae. The most serious adverse effect is encephalopathy in patients co-infected with Loa loa, which can be fatal. Rare effects include hepatitis and Stevens-Johnson syndrome.
Is ivermectin safe during pregnancy?
Ivermectin is pregnancy category C. Animal studies have shown teratogenicity at high doses, so it should only be used if the benefit clearly outweighs the risk. In mass drug administration programs for parasitic diseases, pregnant women are typically excluded from treatment. Discuss risks and benefits with the patient before prescribing.
What is the Mazzotti reaction?
The Mazzotti reaction is an inflammatory syndrome that occurs when treating onchocerciasis with ivermectin, caused by the immune response to dying Onchocerca microfilariae. Symptoms include fever, pruritus, urticaria, lymphadenitis, arthralgias, tachycardia, and hypotension. Severity correlates with microfilarial burden. Treatment is supportive with antihistamines or corticosteroids, and the reaction actually confirms therapeutic efficacy.
Ivermectin vs albendazole — what is the difference?
Ivermectin binds glutamate-gated chloride channels causing parasite paralysis, while albendazole inhibits microtubule polymerization by binding beta-tubulin. Ivermectin is the drug of choice for onchocerciasis, strongyloidiasis, and scabies, whereas albendazole is preferred for most intestinal helminths and tissue nematodes like Toxocara. They are often used together in mass drug administration programs for lymphatic filariasis.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: D11AX22WHO ATC/DDD index: P02CF01
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.