I got a bottle of Mylanta in my Bounty Bag at my first hospital appointment, I was just before 12 weeks... so I thought it would be ok! I have heard otherwise though, and I just stuck to 'preventatives' and Fruit Tingles, Rennies when it got very bad.
Something about the Aluminum in it I think, but I dont know for sure. My HB wasnt extreme so I never really found out about it.