Definitely get the Mylanta Cherry Supreme. No aluminum for babies. Also, the “other” liquid antacids can have salicylates (aspirin) in them, which is also a no-no for babies.
I give my ds who is almost 4 months, 1 tsp for break through reflux. I also give him Mylanta before his last feed at 11 pm, just to be sure he makes it until his 5 am feed. He would probably last that long; but I just don’t want to chance it.
The Mylanta actually helps my ds in the stool dept. He is on Elecare, which can constipate him.