Dependency issues such as milk-alkali syndrome and rebound hyperactivity are among the concerning side effects of antacids, according to RXList. Conditions such as aluminum-intoxication, osteomalacia and constipation are also possible with antacids that include the ingredient aluminum hydroxide.
Nausea, diarrhea and headaches are also common side effects of antacids. More serious side effects include dizziness and fainting, according to WebMD. Very serious side effects include black or tarry stools, shallow breathing, mood changes or confusion, stomach or abdominal pain, or vomiting. Deep sleep or pain during urination are also possible. Someone experiencing these particular symptoms or other problematic ones that persist should contact a doctor immediately.