Except for their slightly constipating effect, antacids are bland and do not cause side effects, at least not immediately. The aluminum-containing antacids, however, may have some undesirable effects on the bones and brain that only show up much later on in life. But even calcium antacids, such as calcium carbonate (the principle ingredient of Tums), which are ordinarily completely safe, are not entirely harmless, according to the Consumers Union News Digest (11:23:9), since they can alter the effects of other medications taken at the same time.
Antacids speed absorption of certain drugs, thereby possibly producing symptoms of overdosage that, in some cases, could be dangerous. Absorption of certain other drugs is delayed by antacids, and this could result in reduced effectiveness. In view of these potential interactions, therefore, anyone who is taking a prescription drug should check with a physician or pharmacist before starting to take an antacid as well.