Problems Curing Pinworms After Missing Dose Of Mebendazole

In our latest question and answer, the pharmacist discusses problems clearing a pinworm infection while taking mebendazole.


Hi, I currently have threadworms (or pinworms) and I am 21 years old. I’ve had them since Sunday night 17th June, I took mebendazole on Monday the 18th. however I’m still finding so many (up to 15 in each poo) many of which are still moving 4 days later. I have had them once before in the beginning of May and I took the mebendazole that time, but I never took the second tablet after 2 weeks to eliminate the eggs. I just wanted to know whether it’s normal for a second infestation to take longer to clear? Also I’m currently taking Marvelon (birth control pill) and I was wondering whether that may have interfered with process of clearing them? I’ve followed all the other steps necessary (cleaning sheets and towels, dusting, hoovering, cleaning bathroom, washing myself). Thank you.

Asked by Ali On Jun 22, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Jul 01, 2018
Blister Pack Pills
  • Your infection is likely taking a little longer to clear because you didn't take your second dose of mebendazole to make sure the eggs from the last infection couldn't survive after hatching. They've survived and you're back to square one, itchier than ever.
  • The other medication, a birth control pill, will not impact the infection or the drug treatments for it. Keep taking that as prescribed.
  • Overdoing hygiene and cleaning stuff is exhausting, frustrating, and not really useful in preventing infection.
  • Mebendazole is 95% or more effective if the second dose is taken, making it extremely useful in preventing infection. But you have to take the second dose. 

Pinworm Infections

Pinworm infection, or enterobiasis is the most common worm infection in humans. It is very treatable, and the treatment is generally pretty effective. Preventing reinfection, on the other hand, can turn into a Sisyphean task if a little planning and taking your medication aren't done.

Pinworms live their entire life cycle (4-8 weeks) inside a single human host. That cycle begins when a person ingests pinworm eggs, usually by touching a surface contaminated with eggs and then bringing their hands to their mouth or using their hands for eating. It has nothing to do with hygiene or being dirty, and isn't something you can prevent from happening. The eggs are extremely durable, countless, and easily stick to everything. Next they hatch in your small intestine, and find their pinworm soul mates at the end of the small intestine. After mating, the males generally die off and are expelled in your stool.

The females migrate and anchor themselves to the lining of your colon, and spend a week or more feeding on colonic contents and generating eggs, enough to fill the worm's entire body.  Gravid ('pregnant') females carry as many as 10-15,000 eggs inside their bodies, and after about 5 weeks post-ingestion, the egg laying process begins. The females book it through the colon at a rate of 12-24 cm/hr (~5-10 in/hr), and emerge from the anus, where they deposit their eggs. Thus the circle of life continues...nature sure is grand, eh?

Pinworms don't lay eggs in your actual stool, but rather in the tissue around the anus. They're kind of impressive in how perfectly, almost diabolically they've evolved to maximize their ability to infect humans. The eggs, which are very sticky, cause intense perianal itching, a crawling sensation, and other irritations that spur the host to scratch the area. You can't see individual eggs, but you may see groups on the tape if you do the clear tape test. You may also see female worms on the surface of your stool, if you have a heavy infection.

Scratching detaches eggs, which can then hitch a ride under the fingernails to almost any other surface. The itching is so intense that it isn't really a question of willpower to refrain from scratching, especially in infected children. You can even scratch in your sleep without ever realizing it.

Treatment Of Pinworm Infections

The primary treatment for pinworms is medication. The hygienic and cleaning stuff is important too, but the eggs are built like tiny nightmare tanks, and are so enormous in number that excessive hygiene/cleaning activities are more likely to be impractical, even counterproductive. The medications used, namely mebendazole and albendazole, are extremely effective, with a cure rate of 95% or more.

So why the heck do you still have pinworms? Because the medication does absolutely nothing to the eggs, since the eggs are deposited outside your body. They can survive for up to 3 weeks in a moist environment (bathroom), or a few days in drier locations (bedding), so reinfection is almost guaranteed even with the craziest cleansing routine imaginable.

Guess when reinfection happens? A week or two after the eggs are deposited. That's why it's very important that a second dose of medication be taken around 2 weeks after the first. This will kill the worms that hatch from these eggs and prevent them from going through their life cycle in your guts all over again.

Non-Medication Treatment Of Pinworms

You're already aware of the non-medication treatment generally offered, like washing first thing upon waking up, cleaning under the fingernails after using the bathroom and before eating, etc. Do this stuff, but don't go crazy. Soap/detergent does nothing to the eggs, and wiping surfaces with bleach or cleaner just spreads the eggs farther. Vacuuming is another unnecessary task that makes the situation worst by kicking up the countless eggs that were settled on surfaces. The eggs will also be launched through the vacuum bag/canister and into the air again, and blown around by the vacuum exhaust. Doesn't sound helpful, right?

Just as important, don't wash yourself to death either. Wash your hands with mild soap and water after you wake up, then after using the bathroom and before eating. For bathing, follow your regular routine, and be kind to your rear end when washing it. Over-scrubbing the anal and/or genital areas with loofahs and soap will leave your skin raw and dry, an opportunity for other non-pinworm infections to get into the mix.

Additional Information

It sounds like you have a pretty good infection going. The worms in the stool are not really helpful in determining the progress of the medication, because mebendazole works pretty slowly. It has to block the formation of microtubules, which the worms to store energy from the colonic contents they digest. Without the ability to store energy, they slowly starve. Another drug option, pyrantel, kills the worms by interrupting their nervous system, and appears as effective as mebendazole as a second-line.

More than anything else, it's super important you take another tablet at 2 weeks, and if you still see signs of active infection (clear tape applied to your backdoor and then removed shows eggs or live adult worms), every 2 weeks until they're gone. If you live with other people, it's very important that they all be treated with mebendazole as well, regardless of whether they show symptoms or not. This drug has few side effects (mild stomach discomfort and diarrhea), and only requires one tablet per dose, two doses two weeks apart. This is because all of them are likely to be infected or uninfected carriers for the eggs.

This isn't fun stuff, but I hope you're feeling better. If you haven't taken your second dose yet, you're just about due based on the date you said you took the first (6/18). I strongly encourage you to take dose 2, and possibly another after that once you've checked with a doctor.

About the Pharmacist

Dr. Randall Higgins Pharm.D

Randall is a Doctor of Pharmacy and drug information specialist. His experience as a pharmacist has taken him from retail to specialty infusion and intrathecal pump management. His interests include pain management (particularly non-opioid), substance abuse, addiction and chemical dependency, and drug/non-drug approaches to these areas. He's also extremely interested in finding better ways to provide people with information on complex and often confusing healthcare topics in a way they can understand and relate to.

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