First of all, the whole IV anti-histamine+Bupe is totally overrated. If you want to shoot bupe, fine, but please don't shoot antihistamines. It DOES NOT get you higher, or make the experience feel way more intense.
Secondly, Loratadine would probably be one of the worst antihistamines to use for this purpose due to the fact that it crosses the blood brain barrier to a much lesser extent than say, diphenhydramine or hyodroxyzine. This is a big part of why Claritan is less sedating than the others I mentioned.
Finally, Injecting even pharm grade ampoules of many antihistamines can cause tissue damage, which is why the ampoules are usually designed for IM (Intra-muscular) injection rather than intravenous injection. I'm fairly certain that Promethazine ampoules have a black box warning for this very reason.