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I'm not sure which over-the-counter antacids are
guaranteed gluten free, but I did just discover
a brand that is available mail order.
 
The company is called Puritan Pride and they
are located in New York.  They make muli-vitamins
and antacids/calcium supplements among other things.
 
I just ordered several bottles of their all natural
chewable antacids with calcium, product #2220.
 
They were pretty reasonably priced, the shipping
wasn't too bad (UPS), and best of all, right now
they have a two for one sale on them, so I will be
getting 10 bottles of 100 instead of 5!
 
I hope this info helps someone.
 
Steve Veach in Chicago, Illinois USA