Yes, that's definitely not within half an hour
All medications have side effects.
In the UK people tend to focus on the risk of stomach upset with Metacam (it's similar to aspirin and ibuprofen so its similar to the risks in people) if used for a significant period of time.
On here most people focus on the fact it can compromise the kidneys, so hydration is very important, again, that's more if its used longerm term.
Personally I have 16 buns, and use Metacam at any sign that something has happened to the bun to cause discomfort to avoid any stasis issues with pain (so this was for all their neuters and spays, tear duct problems, eye infections, when my girl had cancer, any other minor operations, dentals, pre dental, UTIs, infections, irritations or anything else that may cause pain or inflammation). I've never had any problems with mine and you can probably imagine I've used it a lot. I've just always seen more comfortable bunnies after, nothing negative.