The tablet will kill the worms almost immediately - within a couple of days maximum all will be fine. However, Coco pops is right - so many kids have threadworms and it's nothing to do with whether their families have high levels of hygiene etc. My daughter was an 'outdoor' girl when she was younger and bit her fingernails - recipe for disaster my GP told me and she had many bouts until she grew out of them! In all honesty, although they're not nice, they're pretty harmless - it's just one of those things parents are embarrassed to admit their kids have. Well done to your friend for realising it's really not that big a deal and informing you.