@kamama94 On the list that I was given of acceptable and unacceptable OTC products for most patients, the following are acceptable for heartburn:
Pepcid AC
Zantac 150
Prilosec
Prevacid

The instructions for Tums, Mylanta, and Maalox say to take two hours before or 4 hours after transplant medications, so I assume they are safe for kidneys too.

I would suspect these would also be safe for kidneys since those of us on immunosuppressants are prone to kidney problems, that is something they keep a close watch for. I heard of one transplant center that actually told their liver transplant patients that they probably would eventually need a kidney transplant! Thankfully, MGH keeps a very watchful eye and makes adjustments to keep my creatinine down and my kidneys happy.
Of course, you should run these by your own doctor, particularly if you have been told to avoid antacids.
JK

@kamama94 I use peppermint (tea or candy) and ginger (a small piece of candied ginger or tea) for heartburn. I also make sure to keep well hydrated, eat small portions, and eat frequently; this seems to reduce heartburn for me.