Can I take oral antihistamine with me on international trip in unopened box?

Yes. You should be able to take them with you but you might want to check with the airline or a travel agent about peculiar laws the country may have. Any prescriptions you may have, however, will most likely be ok if in a package (bottle/box) with your name and the doctor's name - that would be protected by un charter and the us embassy. Narcotic prescriptions may cause some issues however.