When my 15yr cat started to have difficulties with her rear legs, the vet prescribed metacam, after a week and a bit she went off her food I dosed her from the syringe, rather than add it to her food as my other cat shared shared the food bowls. It would seem from other websites such as ‘metacam killed my dog/cat’ that stomach ulcers are a problem, possibly from not mixing with food. I stopped the metacam and it would appear that muscle weakness was the problem. She lived another six years. Our old Lab had joint problems (no metacam), but it was kidney failure at 13 that got him.
My advice as as a dog/cat owner, NOT a Vet, is to use the metacam and keep a VERY close eye on the appetite and weight.