can anyone advise re: antihistamines?

[festiveface]

ds has a packet of levocetirizine tablets that say to leave 24 hours between doses.
we haven't tried them yet, he has been on benedryl which is twice a day and he last had at tea time yesterday.
question is, do i need to wait 24 hours from the benedryl before i give the new tablets?

I'd think 12 hours, as you just want the Benadryl to have work off, which it presumably does after 12 hours. (And that's assuming the two drugs are similar enough that having them both at the same time would be a problem, which isn't necessarily true. Best to assume it is, until you find out otherwise, though.)