Mebendazole and Constipation
Constipation is not a recognized side effect of mebendazole; the drug is actually associated with diarrhea and abdominal pain, particularly in cases of heavy parasitic infections. 1
Documented Gastrointestinal Side Effects
The FDA-approved drug label for mebendazole explicitly lists the following gastrointestinal adverse reactions 1:
- Abdominal pain (transient, especially with massive infections and worm expulsion)
- Diarrhea (transient, especially with massive infections and worm expulsion)
Constipation is notably absent from the official adverse reaction profile. 1
Clinical Trial Evidence
Multiple clinical studies examining mebendazole safety have consistently documented gastrointestinal side effects without reporting constipation:
In a prospective study of 945 patients (522 children and 423 adults) treated with mebendazole 200 mg three times daily for 3 days, the only gastrointestinal adverse effects reported were abdominal pain (5.6-6.2%), nausea (2.9%), and vomiting (2.3%)—no constipation was documented. 2
A comparative trial in 122 children found that transient abdominal pain was the primary gastrointestinal complaint with mebendazole treatment, again with no mention of constipation. 3
After 5 years of clinical experience reviewed in 1979, mebendazole's safety profile remained characterized by its lack of significant adverse effects, with gastrointestinal complaints limited to those associated with worm expulsion rather than constipation. 4
Mechanism Considerations
Mebendazole's mechanism of action—blocking microtubule systems and inhibiting glucose uptake in parasites—does not affect gut motility in a manner that would promote constipation. 5, 4 The drug's poor systemic absorption further limits its potential to cause motility-related side effects. 4
Clinical Pitfall to Avoid
If a patient on mebendazole develops constipation, consider alternative causes rather than attributing it to the medication. The constipation is more likely related to:
- Underlying parasitic infection effects on bowel function
- Concurrent medications
- Dietary changes during illness
- Dehydration from diarrhea (if present)
The documented side effect profile consistently points toward diarrhea, not constipation, as the bowel-related adverse effect of mebendazole. 1, 2