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A 3-year-old can be given medication for worms once every 3 months if required. Albendazole is the drug of choice. However, if there are no worms or any such complaints you can give it prophylactically once in 6 months.
Albendazole can cause a bit of gastric upset, nausea and vomiting. So it should not be given frequently. Hence once in 6 months is fine. If his hygiene is maintained properly with a good diet, he won't need Albendazole.
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