Threadworm infection is one of the most common parasitic infections in the developed world, and mebendazole is the medicine most doctors and pharmacists reach for first to treat it. This article sets out what threadworm actually is, why mebendazole works against it, what the clinical evidence supports, how the standard 100mg dose is used in practice, and what a family should realistically expect in the days and weeks after treatment.
What Threadworm Is, and Why It Spreads So Easily
Threadworm (Enterobius vermicularis, called pinworm in North America) is a small white parasitic worm, usually less than a centimetre long, that lives in the human large intestine. It is not a sign of poor hygiene or neglect; it is simply extraordinarily good at spreading, because its eggs are microscopic, hardy, and easily transferred from hand to mouth. A female worm migrates out of the anus at night to lay thousands of eggs on the surrounding skin. This causes intense itching, and scratching transfers eggs to fingers and under fingernails. From there they contaminate bedding, clothing, toys, food and household surfaces, and can survive for up to two to three weeks outside the body. Re-swallowing the eggs, or having someone else pick them up, restarts the cycle.
This is why threadworm is genuinely contagious within a household, and why treating only the child who is itching, while leaving siblings and parents untreated, so often fails. It is overwhelmingly common in children of preschool and primary-school age, who share close contact, put fingers in their mouths, and are still learning careful handwashing, but adults living with an infected child are frequently affected too.
Recognising It, and Confirming the Diagnosis
The dominant symptom is perianal itching, typically worse at night when the female worms are active, which disturbs sleep and can cause irritability the following day. In girls, the worms can migrate to the vulva or vagina and cause local itching or discharge. Some children develop mild abdominal discomfort; visible worms, thin and white and slightly moving, are sometimes seen around the anus or in stool a few hours after the child has settled to sleep. Weight loss and significant abdominal pain are unusual and suggest either a heavy infestation or a different cause that deserves medical review.
In most cases diagnosis is made on history and a description of what has been seen, without any test. Where confirmation is wanted, the standard method is the adhesive tape test: a strip of clear tape is pressed against the skin around the anus first thing in the morning, before washing, and examined under a microscope for eggs. Because egg-laying is intermittent, doing this on three consecutive mornings improves accuracy considerably. Routine stool microscopy is a poor way to find threadworm, since the eggs are deposited on the skin rather than passed reliably in faeces.
How Mebendazole Works, and What the Evidence Actually Shows
Mebendazole belongs to the benzimidazole class of anthelmintic drugs, first developed in the 1970s. It works by binding to a structural protein called beta-tubulin inside the worm's intestinal cells, preventing the worm from assembling the internal scaffolding it needs to absorb glucose. Deprived of energy, the worm dies over the following one to three days. Crucially for safety, very little of an oral dose is absorbed into the human bloodstream — well under a tenth of what is swallowed — so the drug acts almost entirely within the gut, where the worm lives, rather than circulating through the body. This is a large part of why side effects are so limited in practice.
The clinical evidence base for benzimidazoles against threadworm is old but consistent. Randomised trials conducted from the 1970s through the 1990s, and summarised in a Cochrane systematic review of interventions for threadworm infection, generally found cure rates with a single dose of mebendazole or a related benzimidazole in the range of roughly 90 to 100 percent at follow-up testing two to four weeks later. That is a strong and biologically plausible effect, consistent across multiple independent trials in different countries, which is why mebendazole has held its place as first-line treatment for decades. The honest caveat is that the certainty of this evidence, by modern standards, is only low to moderate: many of the original trials were small, used varying diagnostic methods, and predate current reporting standards for randomised trials. The World Health Organization nonetheless lists mebendazole among its essential medicines for intestinal worm infections, reflecting decades of consistent real-world experience alongside the trial data. What the evidence does not settle well is head-to-head superiority between mebendazole and the other main options, pyrantel pamoate and albendazole; all three appear to work well, and the choice between them in different countries has as much to do with local licensing and availability as with any clear difference in effectiveness.
Dosing: What Standard Treatment Actually Looks Like
In the United Kingdom and much of Europe, mebendazole is sold over the counter for threadworm, including under brand names such as Vermox, as well as in generic form. The standard regimen for adults and children over two years old is a single 100mg dose taken by mouth, with or without food, and no special preparation, fasting or laxative beforehand.
- Single dose: one 100mg tablet or equivalent oral suspension, taken once.
- Repeat dose: a second 100mg dose two weeks after the first is widely recommended, because mebendazole is more reliable against adult worms than against eggs, and the two-week gap catches any worms that have since hatched and matured, before they can lay a fresh batch of eggs.
- Whole household treatment: most guidance recommends treating everyone in the household at the same time, even those without symptoms, since silent carriage is common and reinfection from an untreated family member is the single biggest reason treatment appears to "fail."
- Age and pregnancy limits: mebendazole is generally not recommended for children under two years old or for use in pregnancy, particularly the first trimester, because safety data in these groups is limited. In those situations, hygiene measures alone, or a different management approach discussed directly with a doctor or midwife, are usually advised instead.
Mebendazole is not approved everywhere in the same way. In the United States, it is available only by prescription rather than over the counter, and pyrantel pamoate is the more commonly used non-prescription option; readers in the US should not assume they can simply buy mebendazole off a pharmacy shelf as they could in the UK or Australia.
What to Expect After Treatment
Mebendazole does not produce instant relief. Itching often continues for several days after the first dose, partly because dying worms and residual eggs on the skin take time to clear, and partly because a genuine allergic-type itch response can persist briefly even after the worms themselves are dead. Most people notice a clear improvement within a week. Because the drug does not reliably destroy every egg, hygiene measures matter just as much as the tablet itself during this window:
- Wash hands and scrub under fingernails after using the toilet and before eating, for every household member.
- Keep fingernails short and discourage nail-biting and scratching.
- Have children wear close-fitting underwear at night and change it each morning.
- Wash sleepwear, sheets and towels in a hot wash on the morning after the first dose.
- Vacuum floors and damp-dust surfaces to reduce settled eggs, particularly in bedrooms.
- Shower rather than bathe in the mornings during treatment, since bathing can redistribute eggs shed overnight.
Where families choose not to use medication at all — for example while awaiting medical advice in pregnancy, or with a child under two — strict hygiene measures alone, sustained for around six weeks to outlast the worm's natural life cycle, can eventually break the cycle of reinfection, but this demands real discipline from every member of the household and is slower and less certain than combining hygiene with medication. If itching, visible worms or sleep disturbance persist beyond a few weeks despite two properly spaced doses and consistent hygiene, or if a young infant, a pregnant woman, or someone with a chronic bowel condition is affected, that is a reasonable point to involve a doctor directly rather than repeating over-the-counter treatment indefinitely.
Side effects from mebendazole itself are uncommon at the doses used for threadworm, given how little of the drug is absorbed. Occasional mild abdominal cramping, diarrhoea or headache have been reported; genuine allergic reactions are rare. It should be avoided by anyone with a known hypersensitivity to it or to related benzimidazole drugs, and anyone on regular medication, pregnant, or breastfeeding should confirm suitability with a pharmacist or doctor before use, as with any medicine given to a family.