"Metacam is a non-steroidal anti-inflammatory drug (NSAID) for use in cats and dogs. It can be used for alleviation of mild to moderate post-operative pain and inflammation following surgical procedures in cats and dogs, e.g. orthopaedic and soft tissue surgery."
The trouble is that with the internet you can find out too much information. I found this...
He is still not weeing normally. I am still worried. He is very loving and woofing down the Royal Canin urinary food they (vets) have sold us. They are saying two and a half sachet's a day (he is 5.4 kg) and I am wondering how he will eat that much as he doesn't normally like wet food but as he is not having dry food maybe he can manage it. Not letting him out at moment so we can monitor weeing.
Metacam is a standard painkiller and anti-inflammatory - both of which properties are necessary to deal with cystitis. And yes, long term use can be problematic, but this is short term, to deal with a short term problem, and should be fine.