What are the adverse effects of ivermectin (antiparasitic medication) at a normal dose?

From the Guidelines

Ivermectin at normal therapeutic doses can cause several negative side effects, though most are mild and transient, with the most significant concern being severe neurological adverse events in individuals with high L. loa microfilaria loads, exceeding 8000 microfilaria/ml, as reported in a study published in 2019 1.

Common Side Effects

• Headache
• Dizziness
• Muscle pain
• Nausea
• Diarrhea
• Mild skin rash Some patients may experience temporary worsening of itching, swelling, redness, or tenderness of the skin, particularly in those being treated for parasitic infections like scabies or onchocerciasis, known as the Mazzotti reaction, which occurs as parasites die and release antigens that trigger an immune response 1.

Less Common Side Effects

• Fatigue
• Abdominal pain
• Constipation
• Weakness
• Drowsiness
• Tremor Rare but serious side effects can include severe skin reactions, liver injury, or neurological effects like confusion or seizures, particularly in those with heavy parasite loads or when doses exceed recommendations, as noted in the 2019 study 1.

Special Considerations

It is essential to exclude loiasis prior to treatment and seek expert advice in loiasis co-infection, as the risk of severe adverse events is high when the microfilaria load exceeds 8000 microfilaria/ml, and the reaction is more severe if the load exceeds 30 000 microfilaria/ml, as reported in the 2019 study 1.

Treatment Guidelines

According to the 2025 guidelines for the investigation and management of eosinophilia in returning travellers and migrants, ivermectin should be used with caution, and the treatment regimen should include doxycycline PO 200 mg once daily for 6 weeks to target symbiotic Wolbachia, and ivermectin PO 200 μg/kg monthly for 3 months to target microfilariae and to reduce micofilarodermia, with repeat ivermectin every 3–6 months until asymptomatic then annually, if necessary, for several years 1.

From the FDA Drug Label

PRECAUTIONS General After treatment with microfilaricidal drugs, patients with hyperreactive onchodermatitis (sowda) may be more likely than others to experience severe adverse reactions, especially edema and aggravation of onchodermatitis Rarely, patients with onchocerciasis who are also heavily infected with Loa loa may develop a serious or even fatal encephalopathy either spontaneously or following treatment with an effective microfilaricide In these patients, the following adverse experiences have also been reported: pain (including neck and back pain), red eye, conjunctival hemorrhage, dyspnea, urinary and/or fecal incontinence, difficulty in standing/walking, mental status changes, confusion, lethargy, stupor, seizures, or coma. Post-marketing reports of increased INR (International Normalized Ratio) have been rarely reported when ivermectin was co-administered with warfarin.

The negative side effects of ivermectin at a normal dose include:

• Severe adverse reactions in patients with hyperreactive onchodermatitis, such as edema and aggravation of onchodermatitis
• Serious or fatal encephalopathy in patients with onchocerciasis who are also heavily infected with Loa loa
• Pain, including neck and back pain
• Red eye and conjunctival hemorrhage
• Dyspnea
• Urinary and/or fecal incontinence
• Difficulty in standing/walking
• Mental status changes, including confusion, lethargy, stupor, seizures, or coma
• Increased INR when co-administered with warfarin 2

From the Research

Negative Side Effects of Ivermectin at Normal Dose

• The studies 3, 4, 5, 6 suggest that ivermectin is generally well-tolerated and has minimal side effects at normal doses.
• Common adverse reactions reported in the studies include:
  • Edema
  • Rash
  • Headache
  • Ocular complaints
• These effects are usually mild and short in duration 6.
• However, it is noted that genetic polymorphisms in P-glycoprotein or coadministration of P-glycoprotein inhibitors may increase the neurotoxicity of ivermectin 6.
• No serious adverse events associated with ivermectin treatment were found in the studies 3, 4, 5.

Comparison with Other Treatments

• A study comparing ivermectin with albendazole for the treatment of strongyloidiasis found that ivermectin was more effective and had fewer adverse reactions 3.
• Another study comparing ivermectin with high-dose albendazole for chronic strongyloidiasis found that ivermectin had a higher parasitological cure rate and no serious adverse events 5.

Mechanism of Action and Toxicity

• Ivermectin's mechanism of action involves binding to ligand-gated ion channel receptors, resulting in parasitic paralysis and death 6.
• The drug's toxicity is rarely reported in mammals due to varying expression of these ion channel receptors in vertebrate species 6.
• Ivermectin is also a substrate for P-glycoprotein, which limits its neurological toxicity in humans 6.