One of the first signs of effectiveness is relief from digestive discomfort. Many people notice reduced abdominal pain, bloating, or nausea within a few days. If worms were causing changes in bowel movements, such as diarrhea or loose stools, these symptoms may gradually improve as the parasites die off. 
Mebendazole 100mg  is an antiparasitic medication commonly prescribed for intestinal worm infections such as roundworm, whipworm, hookworm, and pinworm. While the medicine starts acting soon after ingestion, recognizing whether it is working can provide reassurance.
For pinworm infections, itching around the anus—especially at night—tends to decrease within about a week. Parents treating children may also observe fewer visible worms around the anal area or in stools. In other types of worm infections, the presence of worms or worm segments in stools may briefly increase as the parasites are expelled, which is a normal sign that the medication is working.
General improvements in energy levels, appetite, and weight gain (particularly in children) are also indicators of recovery. Since intestinal worms can interfere with nutrient absorption, successful treatment often restores normal growth and vitality over time.
It is important to remember that symptoms may not disappear immediately. Depending on the infection, a repeat dose or follow-up testing may be needed to ensure complete clearance. If symptoms persist after the recommended treatment period, consult a healthcare provider.
In short, signs such as reduced itching, improved digestion, visible expulsion of worms, and better overall well-being usually indicate that mebendazole is effective