Deworming medications do not commonly cause constipation. The most frequent side effects are stomach pain, nausea, vomiting, diarrhea, and headache. Constipation is possible but rare, reported in roughly 1 to 10 out of every 100 people for certain antiparasitic drugs. In most cases, any digestive symptoms after deworming are mild and resolve on their own within a few days.
What the Side Effect Profiles Actually Show
The three most widely used deworming drugs each have well-documented side effect lists, and constipation barely appears on any of them. Albendazole, the drug most often prescribed for roundworms, hookworms, and tapeworms, lists stomach pain, nausea, vomiting, headache, and dizziness as its recognized side effects. Constipation is not among them. Mebendazole has a nearly identical profile. Pyrantel, the active ingredient in many over-the-counter pin worm treatments, lists stomach cramps, diarrhea, nausea, dizziness, and drowsiness. Again, constipation is absent.
One older antiparasitic called pyrvinium pamoate does include constipation in its known gastrointestinal effects alongside nausea, vomiting, bloating, and diarrhea. Even there, gastrointestinal side effects as a whole fall into the “common” category, meaning they affect somewhere between 1 and 10 out of every 100 people. This drug is rarely used today compared to albendazole and mebendazole. So while constipation is technically possible with certain dewormers, it is not a typical reaction to the medications most people take.
Why You Might Feel Backed Up Anyway
If you’re experiencing constipation around the time of deworming, the medication itself may not be the cause. There are a few other explanations worth considering.
Heavy worm infections, particularly with large roundworms (Ascaris), can physically block sections of the intestine. In severe cases, a mass of worms, living or dead, can occlude the bowel lumen and cause a true obstruction. Case reports describe surgeons finding tightly packed worms in the lower small intestine, sometimes causing the bowel wall to die from the pressure. This is a serious complication that occurs mainly in people with very high worm burdens, often in settings with limited healthcare access. It’s not the same as ordinary constipation, and it comes with severe abdominal pain, vomiting, and an inability to pass gas or stool.
A more subtle possibility involves changes in your gut bacteria. Research on hookworm-infected individuals in Ghana found that a single dose of albendazole significantly altered the composition of gut bacteria within 10 to 14 days. People who successfully cleared their infection saw their gut microbiome shift toward a composition similar to uninfected individuals, with increases in certain beneficial bacterial groups. People who remained infected showed no change. These shifts in microbial balance could temporarily affect digestion and bowel habits, though researchers have not drawn a direct line from this to constipation specifically.
The Constipation Connection in Children
In children, the relationship between worms and constipation can actually run in reverse. A study of Bangladeshi children with recurring pinworm infections found that many of them had underlying constipation, sometimes hidden (called “occult constipation”), that was making their parasite treatment fail. The stool sitting in the lower bowel created an environment where pinworm eggs could survive and reinfect the child even after proper medication.
The researchers found that treating the constipation first, then giving the deworming drug, led to much better outcomes. They recommended that clinicians check for constipation early when children keep relapsing after standard treatment. In other words, the constipation was not caused by deworming. It was already there, and it was making the worms harder to eliminate. Large studies of mass deworming programs in children confirm that adverse effects are infrequent and, when they do occur, tend to be mild and short-lived, typically involving stomach upset and diarrhea rather than constipation.
What to Do if It Happens
If you notice constipation after taking a deworming medication, it will likely resolve within a few days as the drug clears your system. Staying well hydrated and eating fiber-rich foods can help keep things moving. Gentle physical activity also supports normal bowel function during this period.
Watch for warning signs that suggest something more than simple constipation: severe or worsening abdominal pain, inability to pass gas, vomiting, or a visibly swollen abdomen. These could indicate a bowel blockage, especially if you had a heavy worm burden before treatment. This is uncommon, but it requires prompt medical attention. For most people, though, deworming is a short course of medication with minimal digestive disruption, and any changes in bowel habits tend to be temporary.