Threadworms or pinworm infestations occur due to ingestion of parasites present on surfaces like beds/linen. If you have taken mebendazole, then you can expect cure as single dose is sufficient. If infestation persists after 3 weeks, then another dose can be repeated.
One important aspect is all other house members must also be treated even if they are asymptomatic. Mebendazole can be given to children above 2 years without any dose change. For children below 2 years, contact your pediatric consultant. Strict hygienic measures is also equally important. It has to be maintained for around 6 weeks strictly and continued beyond 6 weeks will help in preventing reinfection. Hope the above information clarifies your doubts. Thank you.