Antacids vary in their acid neutralizing capacities, which is their primary function. They can also have highly variable actions on the rest of the bowel. For example, antacid medications containing calcium (Tums) or aluminum (AlternaGel) may have a constipating effect on the bowel. On the other hand, magnesium-only antacids (Mag-Ox and Milk of Magnesia) may have a laxative effect.
When I do recommend antacids, I usually suggest alternating these two types so that the opposing effects on the bowel can offset each other. This can minimize the side effect, while providing you with the required antacid result.
So, for example, if you were taking antacids at four-hour intervals, you might take Tums at 8 am, then Mag-Ox at noon, followed by Tums at 4 am, and Mag-Ox at 8 pm (Of course, if you are taking antacids this frequently, you should consult with a physician. If you haven't already done so, make that appointment!)
An easier option is to take a combination preparation such as Maalox or Mylanta. These over-the-counter medications include opposing ingredients that counteract each other's effect on the bowel. Newer medications such as Pepcid Complete contain not only antacids but also a histamine 2 (H2) blocker (famotidine) medication. Such a combination offers the advantage of acid neutralization by antacid as well as blocking acid secretion by the H2 blocker.