Codeine and Antihistamines

[Tortoise]

Swim usually takes 50mg of dimenhydrinate (Generation I or 'drowsy' antihistamine) half an hour before using codeine to reduce the itchiness that he would otherwise get. This has worked well for him so far, but he'd like to know if it's safe to use a generation II Antihistamine instead (a non-drowsy antihistamine) such as loratadine.

In theory it shouldn't cause any complications but he remembers reading from another internet source (with very questionable reliability) that one should not mix codeine with non-drowsy antihistamines. Is this true? And also, would a non-drowsy antihistamine be any less effective?

Thanks