I have been using Metacam for about 8 years for 3 different dogs. The first dog was on it for the last 3 years of her life, it really helped with her arthritis and she had no side effects that I saw from it. We had graduated to it from a Glucosamine/Chondroitin/MSM mix (though I kept giving this as well to all 3 dogs). When we were giving her the Metacam, it was the only NSAID available for dogs other than injections once a month that were more expensive than the Metacam.
The second dog we tried with several others first, eg: Deramaxx, Previcox and another one I can't remember the name of, they gave her side effects of vomiting and diarrhea that did not resolve after a couple of weeks on each drug. We finally decided that Metacam and the Glucosamine/Chondroitin/MSM mix were the only drugs that she could tolerate without side effects. She remained on the Metacam for 2 years before she passed of old age and complications after surgery.
The third dog was also tried on Deramaxx, then Previcox with the same vomiting and diarrhea side effects that resulted in me finally changing him to Metacam. Again it was the only NSAID I gave him that did not cause side effects. He has been taking Metacam and a Glucosamine/Chondroitin/MSM mix for over a year now and without it he wouldn't be able to use his hind legs to get himself up or walk. He does have a chronic condition that results in days when he misses his meds and the following days show that his hind end weakness is still very much an issue without the NSAIDS.
I am NOT saying that other dogs will not have side effects to Metacam or have stomach problems resulting from its use. Every dog will react differently to any medication given, just as any human may have side effects to a drug they are given. I am only telling you what my own experience with the use of Metacam has been and it has been good for all my dogs, so far.
As for using Petcam, if it were readily available here in North America through my vet or even through an online pharmacy that DOES require a prescription, I wouldn't hesitate to use a generic form of Metacam. However, from googling Petcam it does not appear to be available at an online licensed pharmacy in North America that requires a prescription, so I would really question whether what they were selling was actually Meloxicam or something else. I would wonder if I might be putting my dog's health at risk by ordering a "drug" that could be anything. My hesitation extends to human "prescription" meds available online that do not require a prescription as the same holds true for them.
Believe me I know how expensive Metacam is, but when I think of possibly taking a chance with my baby's health I just can't bring myself to do it. Have you checked with another vet for the price of Metacam as I pay quite a bit less for mine though still over $100 for the 100ml bottle. Good luck it would be wonderful if there were a generic form of Metacam available in North America.