It can cause problems because cats can't metabolise it very easily, but that's not exclusive to metacam, it's common to a lot of painkilling/anti-inflammatory drugs for cats. My Milly was on metacam for about 3 years or so for her arthritis, she had blood tests every 6 months to check for organ damage but fortunately she was fine. Milly has been having acupuncture and we have now managed to get her comfortably off the metacam which is great, as I was a bit concerned about it for such long term use.
Alas painkillers plus cats aren't a great combination so in many instances it does need to be a balance of what the cat needs to control pain/inflammation v the potential risks. For a few weeks, personally if the vet thinks the cat needs it, then I would be happy to do that provided I didn't see any worrying signs in the cat during that time - you're always told to stop immediately and seek veterinary advice in the event of vomiting etc. it may be unrelated of course, but better safe than sorry.
Hope your little one is ok xx
