We haven’t used it but have used Deramaxx and Rimadyl which come with similar concerns, although Metacam is said to be even safer. My vet told me (specifically to the Rimadyl) that when given for two weeks or less there isn’t really any reason to worry (although of course keep an eye on your dog for immediate drug reactions). If the dog will be on it longer than that is when they run blood tests to make sure liver function, etc., isn’t being affected. While liver damage is the most serious concern, the most common reactions to these types of drugs is actually gastrointestinal upset, so if you see signs of nausea, vomiting, loss of appetite, etc., stop the drug and call your vet.
That said, Scully was on Deramaxx for 7+ years with no problems; Mulder was on Rimadyl for almost 2 years with no problems, and Chloe has done several short courses of Rimadyl, also with no problems.