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

Mebendazole (brand name: Emverm) is generally well-tolerated, particularly with the short treatment courses typically prescribed for common intestinal worm infections like pinworm. Because mebendazole is poorly absorbed into the bloodstream — it acts almost entirely within the digestive tract — most side effects are gastrointestinal and temporary. However, serious side effects can occur, especially with high doses or prolonged use. This guide explains what to expect and when to reach out to your doctor.

Common Side Effects of Mebendazole

The most frequently reported side effects of mebendazole — particularly with short-course therapy — are mild and gastrointestinal in nature. They typically appear within hours of taking the medication and resolve within a day or two. These include:

• Abdominal pain or cramping — Often occurs as worms die and are expelled from the gut
• Diarrhea — Usually mild and short-lived
• Nausea — Can occur after taking the tablet
• Vomiting — Less common but possible
• Flatulence — Increased gas is commonly reported
• Loss of appetite (anorexia) — Usually temporary
• Rash — Mild skin reactions have been reported

These side effects were documented in a safety evaluation of mebendazole across 39 clinical trials involving more than 6,276 adult and pediatric subjects. They are generally manageable and resolve without additional treatment.

Serious Side Effects — When to Call Your Doctor Immediately

Serious side effects from mebendazole are rare with standard short-course dosing but can occur — especially with high doses or prolonged treatment (such as for echinococcosis). Contact your healthcare provider immediately if you experience any of the following:

• Signs of low white blood cell count (neutropenia/agranulocytosis): Fever, chills, sore throat, mouth sores, unusual fatigue, or easy bruising or bleeding
• Signs of liver problems (hepatitis): Upper abdominal pain, dark urine, yellowing of the skin or eyes (jaundice), severe fatigue
• Severe allergic reaction (anaphylaxis): Difficulty breathing, swelling of the face, lips, tongue, or throat, severe rash — call 911 immediately
• Severe skin reactions (Stevens-Johnson syndrome/TEN): Painful skin blistering, peeling, sores around eyes, nose, mouth, or genitals — seek emergency care immediately. Risk is highest when mebendazole is combined with metronidazole.
• Seizures (in young children): Convulsions have been reported in post-marketing surveillance for infants under 1 year. Mebendazole is not approved for children under 2.

Side Effects with Long-Term or High-Dose Use

Most people take mebendazole for just 1 to 3 days for common intestinal infections. But for conditions like cystic echinococcosis, longer treatment courses at higher doses may be used. With prolonged therapy, additional risks include:

• Elevated liver enzymes (AST, ALT, GGT) — periodic liver function tests are advisable
• Neutropenia or agranulocytosis — periodic CBC monitoring is recommended
• Hair loss (alopecia) — reported with high-dose extended therapy
• Glomerulonephritis (kidney inflammation) — rare, reported in post-marketing surveillance

Special Populations: Pregnancy, Children, and the Elderly

Pregnancy: Mebendazole is classified as FDA Pregnancy Category C. It has shown embryotoxic and teratogenic effects in animal studies. It should be avoided in the first trimester. Use in the second or third trimester may be considered if the benefit clearly outweighs the risk. If you are pregnant and have an intestinal worm infection, discuss the timing and choice of treatment with your OB-GYN.

Children: Mebendazole is approved for children 2 years and older. Use in children under 1 year is contraindicated due to reports of convulsions. Limited safety data is available for children under 2 years.

Breastfeeding: A small amount of mebendazole is excreted in breast milk. Limited case series suggest no adverse effects in nursing infants. Most guidelines consider mebendazole compatible with breastfeeding, but consult your provider.

Key Drug Interaction Warning: Metronidazole

One important interaction to be aware of: combining mebendazole with metronidazole (Flagyl) can significantly increase the risk of Stevens-Johnson syndrome and toxic epidermal necrolysis — severe and potentially life-threatening skin conditions. This combination should be avoided. Always tell your doctor and pharmacist about all medications you're taking before starting mebendazole.

For a complete list of drug interactions, see our guide to mebendazole drug interactions.