I'm visiting Japan in a few weeks and, after
reading information on the internet regarding
certain medicines being banned in the
country, I was wondering if anyone enlighten
me as to whether I would be able to bring
Clarityn tablets with me?

If not, would I be able to buy antihistamine or
allergy tablets in Japan?

I don't think there is any problem with antihistamines. The main over-the-counter medication you could have problems trying to bring in is any kind of cold medicine with (I think) pseudoephydrine or something like that which can easily be reprocessed to make speed, or any pain medication with codeine in.

I've brought antihistamines in many times with no problems. You can also buy antihistamines here, but the ones available over-the-counter here tend to be the older-generation kind which make you drowsy; for more effective ones you still need a prescription, and if you're just on a short visit you won't want to do that.