DON'T BELIEVE ALL THE HORROR STORIES THAT SAY "ONCE YOU GET THREADWORMS YOU HAVE THEM FOREVER". THIS IS SIMPLY NOT TRUE.
I thought I had threadworms 3 times, and I spent several months of my life going crazy trying to get rid of them. It turns out I was actually making the problem worse, and I'm so glad I found out the real cause, because otherwise I could have carried on that way forever. Here are some things I learned that I think are worth sharing, so hopefully other people don't go through the same thing I did.
Signs you might not have threadworms:
1) You feel them crawling during the day and not just at night.
2) Other people in your family don't catch them or don't feel them.
3) You have repeatedly taken the medication and been through the hygiene measures and it seems to have little or no effect.
4) You have not actually seen a definitive living threadworm, and you have tried doing tape tests, for example even right after you feel them crawling.
In my case, the cause of the crawling feeling was actually overwashing. I expect this is common problem in association with threadworms. See, I did originally have a threadworm infection (a year and a half earlier), and that made me become obsessive about washing my bottom. After reading the horror stories here, I wanted to make sure I completely got rid of them. I would spend at least a couple of minutes just washing around my anus to make sure no threadworm eggs got left behind. It turns out that washing so thoroughly, with soap, makes the skin in this area super-sensitive, and it starts tickling occasionally for no reason. And that tickling feels EXACTLY like threadworms. I swear it is indistinguishable.
A conversation I had with my mum, when I still thought I had threadworms and she suggested that it might just an itch. I responded with: "It's not an itch, I can feel it crawling out and across my skin. I KNOW what threadworms feel like. I first found out I had them BECAUSE I could feel them, it was only afterwards that I saw one. I've never had that feeling before then, ever in my life. So I know this is them again, because there's nothing else that would cause a sensation like that."
Turns out I was wrong! It honestly can feel like they are crawling 'out and across your skin' even when they are not.
Other thoughts:
* All the scientific studies say that threadworms come out only at night, e.g. after 9 pm. I believe this is correct.
* A few people have asked how quickly mebendazole works, and how effective it is. I don't know for definite, but I personally believe it kills worms quickly (within 5 days), and is probably close to 100% effective. This is because my own feelings of having worms changed from nighttime-only to daytime within a few days, and so I think it is quite common for the over-sensitive skin thing to kick in quickly once you start washing to get rid of the threadworms.
What I would do if I ever thought I had threadworms again:
1) take the mebendazole once a week for a few weeks (6 at maximum)
2) make sure I don't wash too thoroughly after the first few weeks, and get an emollient-based soap-substitute (available from a pharmacist, usually used for eczema).
3) go and see a doctor if it didn't go away.
My problem went away within a couple of days after I stopped over-washing.
I hope this helps other people, and if it does, please, PLEASE come back and post to say so. Or if it doesn't work, or you disagree with me, or you don't believe me, you're welcome to post that too. I feel like this forum has become over-run with scare stories and unhelpful advice, and nobody seems aware of other possible causes for threadworm-like symptoms.