If we take a base to neutralize the effect of HCl in our stomach, neutralization reaction will take place which is an highly exothermic reaction. How it doesn't causes us any harm?
When the stomach produces too much of acid, it results in digestive disorder. This causes pain and irritation, or what we commonly call acidity. To treat acidity, antacids are used. These antacids are mild bases which neutralise the acid. The antacid most commonly used is milk of magnesia. In the body it reacts with HCl to form salt and water. Not very large amount of heat is produced by this reaction because :
1) the base used is very mild. Hence not much energy would be released.
2) Very small amounts of antacids are required. A small dose of 0.5-1.5 g in adults works very effectively
3) water is produced in the reaction which absorbs the heat.
The heat produced by such reactions in the body is very small as compared to energy released during other metabolic processes that take place in the body. Hence the body can easily withstand the energy produced during this reaction and hence these antacids cause no harm.