When discussion threads like these pop up, I occasionally take advantage of them to discuss what is allowed, what is approved, what is 'extra-label', and what is prohibited. I see recommendations on here, all the time, of 'give this', or 'give that'... and in some cases, those are good recommendations... other times, they are bogus BS from people who don't know the drugs and their modes of action, the condition at hand, or whether the thin they're recommending will really even work.
Kenny hit the nail on the head... misuse of some drugs in the past have led to them being prohibited. In some cases, it's just been a case of increasing knowledge base that has lead regulatory authorities to ban them.
Banamine (flunixin meglumine) - which may be the 'B' drug the OP's vet prescribed - is approved for use in cattle as an intravenous injection.
How many folks on this board, some of whom seem to use it like it was water, give it IV? It can be given intramuscularly, but that is an 'extra-label' usage, and projected meat withdrawal time changes from 10 days for IV administration to 60+ days for an IM injection. Multiple doses? Go past 60; Do not collect $200.
Banamine tissue residues are one of, if not THE top offender for drug residues in meat... mainly due to its widespread use in dairy cattle, with cull dairy cows going to slaughter every day... there are some 'frequent flyers' who are not allowed to sell cattle, because they've had so many violative residue incidents.
Yeah, lots of people give it IV because they can't or don't want to hit a vein. It's not innocuous... it does cause some muscle damage at the site of injection.
Bute(phenylbutazone) pretty much HAS to go IV (unless you're giving oral product); if you give it IM, you'll have a nasty necrotic mess on your hands.