Like Lillith I've never heard of lipomas as a side effect of Proin, and I do know some dogs are just prone to them. My first Rottie developed quite a few and probably started about the age of your dog. I had a needle biopsy done on each one as I discovered it. If all that shows is fatty cells, you can be fairly sure it's a lipoma. That girl wasn't on Proin and 2 that were never had fatty lumps even in old age. The Rottie I have now has one I discovered a couple years ago and hasn't developed more. She's 7 now. A friend had a mixed breed male dog that was covered with lipomas starting at a pretty young age.