kutyuska ,
Pain meds can cause problems with the stomach for sure, I hope this is better soon.
If you suspect arthritis is part of Ace's discomfort , I would consider a joint supplement. it will not upset the stomach and it has helped my dog that has joint pain.
Supplements to the dog's diet such as Glucosamine and Chondroitin Sulfate, Omega Fatty Acids, Vitamin E, Selenium, and MSM (Methylsulfonylmethane) all assist in decreasing inflammation and improving the body's ability to repair and strengthen tissues.
Example: Cosequin® DS Plus MSM Joint Health Supplement for Dogs , this product is used by many of the folks I know with proven results for dog joint pain...
I buy the Glucosamine and Chondroitin (500/600 mg ea) at the grocer store and add a generic fish oil gel cap to my dogs food. Takes about 2 weeks to start seeing results.
Make sure that the dogfood you are using has a little vitamin C in it as this help the body absorb the glucosimine/chondroitin.
You could try this as a test and it may help Ace.
Best , oldhounddog