There is a lot of bad advice on threadworms. The kids have had them a few times over the years, and the first time it took me ages to work out how to break the cycle.
The first dose of medbendazole will kill existing worms within 2-3 days, however you must follow up with a second dose to kill worms that have hatched from eggs swallowed around the time of the first dose. the advice in the pack is to take the second dose after two weeks. From checking my kids bottoms, the new batch of worms start coming out to lay new eggs after 11 days not two weeks. So you must do the second dose 10 days after the first dose. Otherwise you will already be swallowing new eggs laid by the new worms on days 11-14 so starting the cycle all over again I think this is why so many people have trouble clearning them - they are either not bothering with a second dose or taking the seocnd dose too late.
Good luck.