I'm glad you are continuing to find ways to manage his condition including the dosing of the medicines. I've found that to be very important with all of the meds for Cinnabon. I've been charting it out and really thinking about the effects of each and when the optimal time to give each one for maximum benefit or to minimize side effects.
It's definitely draining at time caring for these aging dogs, at least it is for me.
About the doxy and niacinamide, ChiChi was on both of those for her autoimmune issues. She took the doxy for almost 7 years, and while it can be upsetting to the GI tract like any antibiotic, she never had a problem with it. I gave it with a meal or a small amount of food, and some of the time I didn't give probiotics either. At the end of her life she did have some ulcers in her mouth and probably in the esophagus and maybe the stomach, but that also could have easily come from the increaseing kidney values too and not necesarily the doxy.
She was also on the niacinamide until she developed all the microfractures of the cornea over the entire surface of both eyes that was from dry eye issues, and I found an article that said that a side effect in humans is dry eyes. I just did an online search and found the site - it was from the Mayo Clinic. It also says that it can cause inflammation of the cornea. Of course, my (old) regular vet thought I was crazy and wouldn't listen to anything I said if it was info I'd read on the internet and snottily asked where I'd read that tidbit. There was a lot of silence from her when I told her I knew to read only reputable sites and where I'd found the info. She still didn't believe it even after I dropped off a printout of the article. Thank goodness I had a great opthalmologist. I don't remember if he agreed that the niacinamide was the culprit for her eye troubles or not, but we decided to stop using that and see if her AI problems got worse without it, and they didn't so I never started it up again.
Here's that page, if you are interested: http://www.mayoclinic.org/drugs-supplem ... B-20059838