I'm assuming your vet prescribed the Metacam for your cat?

Cosaquin is a MUCH safer product to be using in cats than Metacam. My Dusty had been on Cosaquin for several years and not only did it help her osteoarthritis alot but she loved eating it!! You have to understand Dusty was the pickiest cat on the planet preferring to litterally starve herself for 3 or more days rather than eat anything with meds in it she didn't like! That is not an exaggeration, she had baffled several vets with her unorthodox behavior in the past.

Cosaquin comes in nice capsules that you break open and sprinkle on the food (I put it on a bit of wet food for Dusty every morning). No pill splitting or crushing like with the Restor-a-Flex we had been giving. And Cosaquin worked much better for her. Cosaquin is a glucosamine/chondroiton supplement formulated specifically for cats (although there is a dog formula out there too).

Up until the last few months of Dusty's life when we had to resort to Metacam injections and Tramadol pills (neither of which are approved for use in cats) we had great success with the Cosaquin and I'd highly recommend it to anyone. Dusty was 17 when we had to have her PTS this past July.

As far as weight loss goes 25 lbs is a great deal of weight. While 10 years old isn't ancient as far as housecats go, the cat could be getting the start of arthritis particularly with that excess weight. Extra weight adds stress to the joints and seriously increases the risk of diabetes and other weight related diseases.

What kind of food/treats is the cat getting now? If you are gone long periods of time during the day and the other cat is a nibbler I'd highly recommend trying to keep the cats separated when you aren't home so the nibbler has access to the food but the fat kitty doesn't.