Absolutely they are real rough on your stomach. I have taken them plenty in my life time. I just finished two rounds for a sinus infection. It caused my acid reflux to flare up again. So then I had to take some prilosec. It is a delicate balance. I think I may still have a touch of a sinus infection. The doctor doesn't want to prescribe more antibiotics right now and try some natural things.
I have done some research and have started a program of taking oil of oregano. It is wonderul. I take 4 drops in a shot glass of water and hold it under my tongue. It is natural and kills yeast in bacteria in your body. It is an anti fungal. Sinus infections are fungus, so this is an excellent product for these problems. I wish I had found out about it years ago. It hasn't bothered my tummy although they recommend that you have something in your stomach while taking that.
Hang in there! Those antibiotics do destroy the good flora in your tummy and digestive track. You may want to eat yogurt and take a probiotic. Perhaps try the oil of oregano if you are up for it! Good luck!