I gave my son one 100 mg tablet for pinworm. He itches again on day 12. Do I start a three-day course?
Start with the 100 mg chewable
One strength, two clocks
US tabletEMVERM 100 mg chewable, Rx
Age on the file2 years and older
How to takeChew, swallow, or crush with food
Pinworm clock1 tablet once
Other listed nematodes100 mg morning and evening x 3 days
EMVERM is a 100 mg chewable tablet. It is a prescription medicine in the United States. The same milligram load is used for adults and for children two years and older. Do not invent a mg/kg split for this product.
You may chew it, swallow it, or crush it into food. No fasting. No purge. A fatty meal can raise the tiny absorbed fraction. The anthelmintic job still happens mostly in the gut lumen, so food is not a ritual the way it is for albendazole tissue courses.
The listed gut infections are Enterobius vermicularis (pinworm), Ascaris lumbricoides (roundworm), Trichuris trichiura (whipworm), and the hookworms Ancylostoma duodenale and Necator americanus. That is the whole US indication sentence.
Safety and effectiveness of this 100 mg chewable are not established under two years. Convulsions have been reported with mebendazole in children under one year. Those lines are why a parent of a toddler needs a clinician, not a leftover tablet from an older sibling.
A different mebendazole product, a 500 mg chewable used in some mass programmes, is not this walk. WHO preventive chemotherapy often uses a single 500 mg mebendazole or 400 mg albendazole in endemic schools. That is public-health dosing. It is not the EMVERM pinworm instruction.
Keep the chewable in the gut, not in a tissue course
After 100 mg twice daily for three days, plasma mebendazole stays at or below 0.03 mcg/mL. The main metabolite stays at or below 0.09 mcg/mL. Most of the dose never leaves the intestine. That is why this tablet is a lumen drug.
Protein binding of the absorbed fraction is 90 to 95%. Volume of distribution is 1 to 2 L/kg. Metabolites have no anthelmintic activity. Liver impairment can raise plasma levels, which is a caution, not a reason to use mebendazole as a brain-cyst drug.
Hydatid disease and neurocysticercosis are albendazole's US labeled tissue jobs, taken with food, with blood counts and liver enzymes on a calendar. Mebendazole 100 mg is the wrong product for those cysts. The comparison page walks that fork.
Published pregnancy series have not shown a clear pattern of major birth defects. The label still discusses risks of untreated helminths in pregnancy and limits of those studies. First-trimester decisions stay with the obstetric clinician.
High-fat food can raise the small absorbed fraction. The label still says the amount left in the gut, where pinworm and Ascaris sit, should not change much. So a child can crush the tablet into yogurt. That is not a reason to chase tissue levels with extra 100 mg tablets.
Impaired liver function, slow metabolism, or blocked bile flow can push plasma mebendazole higher. That is a monitoring note for people with liver disease, not a license to treat a cyst. If the imaging word is cyst, stop this chewable walk and open the albendazole file.
Repeat after two or three weeks
CDC's pinworm page says give a second dose two weeks after the first. The medicines do not wipe eggs. Newly hatched adults refill the cycle if you stop at day one.
EMVERM's own label uses a slightly different trigger: if the patient is not cured three weeks after treatment, give a second course. That is why this walk says 2 to 3 weeks, not a single magic Saturday.
In practice many US clinicians follow the CDC two-week repeat for everyone in the house, because waiting to 'see if it worked' is how a classroom returns the worm. The label's three-week line is the fallback if symptoms persist after a first course.
Treat infected people, caretakers, and household members on the same two-dose plan when more than one person is infected or when infections keep bouncing. Institutions can mass-treat and repeat at two weeks.
Hygiene is the other half of the repeat. Morning shower, trimmed nails, hand wash after toilet and before food, hot wash of underwear and bedding. The chewable cannot mop a mattress.
Treat the household on the same day
Pinworm eggs hitchhike. CDC's plain instruction is to treat the household and caretakers with the same two-dose plan when spread is likely. One child chewed, three siblings skipped, is a relapse plan.
Same-day timing beats a staggered weekend in which the untreated adult keeps shedding eggs onto the couch. Write the second date on the bottle when you give the first chewable.
Underwear, pajamas, and bed linen go through a hot wash. Fingernails get cut. Hands get washed. These steps sound fussy because they are the actual transmission route.
Schools and day care see repeat waves. Mass treatment plus a two-week repeat can work in an institution. A single secret tablet in one backpack does not.
Sexual contacts who share anal-oral exposure are part of the same epidemiology in adults. Treat the network you actually have, not the pediatric cartoon only.
CDC also flags caretakers, not only blood relatives. A grandparent who bathes the child and a babysitter who changes sheets belong on the same two-dose list when infections keep returning. Write names on a scrap of paper the day the first chewable goes out. People forgotten on day one become the reservoir on day 16.
Bathing after the morning tape, then clean underwear, cuts the egg load that fingers carry to the mouth. That is boring. It is also the step families skip while they argue about brands.
Give one tablet for pinworm
For enterobiasis the EMVERM table is one 100 mg tablet, once. CDC writes the same first swallow. Pyrantel pamoate and albendazole are other pinworm options. Only pyrantel is sold without a prescription in the US.
The drug kills worms. It does not reliably kill eggs. That single fact is why a one-tablet 'cure' story keeps failing in households with a sandbox and shared towels.
Diagnosis is often a morning tape test or a parent seeing thread-fine worms at the anus at night. Vague belly pain without that picture is a weak reason to start a benzimidazole. Confirm, then chew.
Pregnancy and children under two need a clinician to weigh untreated infection against the product. CDC tells clinicians to do that weighing rather than copy the adult chewable downward.
If the itch is pinworm, treat on the day you have the tablet in hand. Waiting a week for a 'more convenient Saturday' leaves a week of eggs on pajamas.
Tape is better first thing in the morning, before a bath. Seeing a live thread at night is enough in many homes. A random afternoon stool often misses Enterobius, because the females lay on skin, not in the bowel lumen you sampled.
Do not run a three-day nematode course 'just in case' after a tape-positive pinworm. Extra days do not kill eggs any better. They only empty the bottle you need for the timed repeat.
Use morning and evening for three days when the worm is not pinworm
| Infection | First 100 mg action | Repeat logic |
|---|---|---|
| Pinworm (Enterobius) | 1 tablet once | CDC: 2 weeks. Label: 2nd course if not cured at 3 weeks |
| Roundworm (Ascaris) | 100 mg morning and evening x 3 days | Label: 2nd course if not cured at 3 weeks |
| Whipworm (Trichuris) | 100 mg morning and evening x 3 days | Same 3-week second-course line |
| Hookworm (Ancylostoma or Necator) | 100 mg morning and evening x 3 days | Same 3-week second-course line |
For Trichuris, Ascaris, and hookworm the chewable is taken twice a day for three days. That is six 100 mg tablets, not the single swallow used for pinworm.
The same three-day clock applies to adults and to children two and older. Do not stretch a leftover pinworm pack into a half course because the box ran short.
If the patient is not cured three weeks later, the label again advises a second course. Stool follow-up matters more here than it does for garden-variety pinworm, because burden and species vary.
Ivermectin is not the first US tablet for these four gut nematodes. Strongyloides is ivermectin's labeled gut worm, and that is a different species with a weight-based 3 mg count. See the ivermectin uses cut if the lab said Strongyloides, not Ascaris.
People sometimes want a single 500 mg mebendazole swallow because WHO school rounds use it. That is a programme tablet and a different strength. Do not assume a US 100 mg bottle contains that public-health regimen.
Stop when the hygiene loop is closed
A second itch at day 10 is often eggs, not a weak 100 mg tablet. Finish the CDC or label repeat. Wash the house. Do not stack random extra chewables between those dates.
If the lab named Strongyloides, river blindness, or scabies, you are off this chewable. Use the ivermectin monograph and the uses cut. If someone is shopping three anthelmintics by brand, use the ivermectin, mebendazole or albendazole page.
Chew 100 mg on the clock that matches the worm. Then stop inventing a fourth clock.
If two children share a bedroom and only one was tape-positive, still treat both on the CDC two-dose pinworm plan when the house keeps cycling. The 100 mg chewable is cheap next to another month of night itching. It is not cheap if you spend it on a three-day course the slip never asked for.
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What readers asked
Answered by Dr. Elena Marchetti, MD · Infectious diseases & clinical pharmacology
Parents write after the first 100 mg chewable, when the itch returns or a sibling was left out. The answers stay on EMVERM and CDC clocks.
No. Day-12 itch after pinworm is the reason CDC already booked a second single 100 mg swallow at two weeks. Eggs hatched. You do not convert pinworm into the roundworm three-day schedule unless a lab named Ascaris, Trichuris, or hookworm. Give the second 100 mg, treat the household on the same day if you skipped them, and run the laundry and nail steps. The EMVERM label also allows a second course if he is not cured at three weeks. That is still one tablet per course for pinworm, not morning-and-evening for three days. If tape tests stay positive after two properly timed single doses plus hygiene, then we look again at diagnosis and at whether an untreated contact is reseeding the house.
The stool report says Ascaris. I only have two 100 mg tablets left from a pinworm script. Is that enough?
It is not enough. Ascaris on the EMVERM table is 100 mg in the morning and 100 mg in the evening for three days. That is six tablets, same for an adult or a child two years and older. Two leftover pinworm tablets are a third of the course. Stopping early is how people later say 'mebendazole failed.' Get the remaining four tablets prescribed. Check stools after, and read the label's three-week second-course line if they are still positive. Do not substitute a guessed ivermectin count. Ivermectin's US gut indication is Strongyloides, not Ascaris. And do not crush a veterinary paste into the gap. Wrong product, wrong species.
Can I buy mebendazole 100 mg at the pharmacy without a prescription, like the pinworm liquid my mother used?
In the United States the 100 mg chewable (EMVERM) is prescription. The over-the-counter pinworm drug CDC names is pyrantel pamoate, dosed by weight (11 mg/kg, maximum 1 g), then repeated in two weeks. Albendazole is also used for pinworm at 400 mg once, repeated in two weeks, and it is prescription here, not a shelf bottle. Other countries sell benzimidazoles differently. Do not import a foreign OTC story onto a US checkout. If you cannot see a clinician quickly, pyrantel is the legal no-Rx option for pinworm, still with the household and hygiene plan. For roundworm or hookworm I would not start from an OTC liquid meant for Enterobius. Those nematodes need the three-day 100 mg clock or a clinician's albendazole plan, not a pinworm liquid guess.
My partner is pregnant. The school sent a pinworm letter. Do we chew 100 mg anyway?
Do not automatically chew the leftover tablet. CDC asks clinicians to weigh drug and infection in pregnancy and in children under two. Pyrantel is often the agent obstetric teams prefer for pinworm when treatment is needed, but that is an individual call. Untreated soil-transmitted helminths can harm mother and fetus. That sentence is on the mebendazole label as context, not as an order to self-dose in the first trimester. Treat non-pregnant household members on the CDC two-dose pinworm plan so the home stops circulating eggs. Then the pregnant partner's clinician picks the agent and the timing. Bring the school letter and any tape-test result to that visit. If the letter only says 'worms,' ask whether they mean Enterobius. A three-day Ascaris course is a different clock. A group-chat vote is not an antenatal plan.
Treat every answer here as general teaching, not a decision made for the individual who wrote in. What is right for you turns on your history, your bloods and the rest of your medicine list — and that is a conversation for a prescriber who can see all of it at once.