Ivermectin Side Effects: What to Expect and When to Call Your Doctor

When taken as prescribed by a licensed healthcare provider, ivermectin is generally well-tolerated. It has a decades-long safety record across hundreds of millions of doses, primarily in the context of global antiparasitic programs. That said, side effects do occur, and there are important warning signs to watch for. Here's what you need to know.

Common Side Effects of Ivermectin

These are the most frequently reported side effects at recommended doses. They are usually mild and resolve on their own:

• Nausea: One of the most common side effects. Taking ivermectin on an empty stomach (as directed) is important; food can increase absorption and raise blood levels, potentially worsening side effects.
• Diarrhea: GI upset is common, particularly in the first 24 hours after dosing. Usually self-limiting.
• Dizziness or lightheadedness: Particularly upon standing (orthostatic hypotension). Avoid driving or operating heavy machinery until you know how ivermectin affects you.
• Drowsiness or fatigue: Mild sedation can occur. Plan your dose for a time when you don't need to drive or perform tasks requiring full alertness.
• Itching or skin rash: Particularly when treating onchocerciasis (river blindness), a skin reaction called the Mazzotti reaction occurs as dying parasites release inflammatory compounds. This is expected and not necessarily a drug allergy.
• Headache: Mild headaches are reported, particularly in the first day after dosing.
• Fever, joint or muscle pain: Part of the Mazzotti reaction during onchocerciasis treatment. Antihistamines and aspirin are typically sufficient to manage mild to moderate reactions.

The Mazzotti Reaction: What It Is and What to Expect

If you're being treated for river blindness (onchocerciasis), you may experience a set of inflammatory symptoms in the first 1-4 days of treatment. This is called the Mazzotti reaction and is caused by your immune system responding to dying microfilariae (parasite larvae), not directly to the drug. Symptoms include:

• Fever, chills, and sweating
• Swollen, tender lymph nodes (in neck, armpit, or groin)
• Eye redness, swelling, or vision changes
• Skin rash, itching, and hives

Mild to moderate Mazzotti reactions are expected and can be managed with antihistamines, aspirin, or acetaminophen. Severe reactions should be reported to your doctor immediately.

Serious Side Effects — Seek Immediate Help

While serious side effects are uncommon at prescribed doses, they do occur and require prompt medical attention:

• Neurological symptoms: Confusion, disorientation, difficulty walking, seizures, loss of consciousness, or coma. These are rare but can occur — particularly with overdose, use of veterinary formulations, or drug interactions that increase ivermectin's CNS penetration. Call 911 immediately.
• Severe allergic reaction (anaphylaxis): Hives, difficulty breathing, swelling of the face, lips, tongue, or throat. Call 911 immediately.
• Severe skin reactions: Stevens-Johnson syndrome or toxic epidermal necrolysis (peeling skin, blistering) — rare but reported. Contact your doctor immediately if you develop unusual skin blistering or peeling.
• Liver effects: Elevated liver enzymes or bilirubin (usually detected on lab tests, not symptomatic in most cases).

Who Is at Higher Risk for Side Effects?

• Patients taking warfarin: Ivermectin can significantly increase the anticoagulant effect of warfarin, raising bleeding risk. Warfarin users need close INR monitoring after taking ivermectin.
• Patients taking P-glycoprotein inhibitors: Drugs like quinidine, ritonavir, or itraconazole can increase ivermectin levels and its CNS penetration, raising the risk of neurological effects.
• Patients co-infected with Loa loa: In patients with high Loa loa parasitemia (common in parts of West and Central Africa), ivermectin can rarely cause a serious encephalopathy. This is why travel history is important for providers to know.

For a full breakdown of drug interactions and contraindications, read our guide on ivermectin drug interactions.

What Not to Do: Veterinary Ivermectin

Veterinary ivermectin products are highly concentrated formulations designed for large animals (horses, cattle). These are NOT safe for humans. Taking them can cause severe overdose with nausea, vomiting, low blood pressure, seizures, coma, and death. Poison control centers saw a sharp increase in calls related to veterinary ivermectin ingestion during the COVID-19 pandemic. Do not use veterinary ivermectin under any circumstances.