Stomach

Honestly gastritis is very common to even people without cancer and chemo treatment. We do however take time to recover. I needed nexium twice a day to settle mine on chemo. I was one to have  gastritis well before treatment (especially when stressed or after tummy bugs)  but it only got worse. Your Dr could prescibe something and you still discuss with the oncologist. You likely will need some tests eventually. Zantac on its own doesn't provide long enough cover but can be useful as an add in on a proton pump inhibitor like nexium. 

I'm awaiting a scope just to check on it. They weren't much interested but I requested since I now have dairy food issues. 

This link gives an overview of gastritis.

https://www.mayoclinic.org/diseases-conditions/gastritis/symptoms-causes/syc-20355807