Hydrochloric acid (HCl) insufficiency can be complete (achlorhydria) or partial (hypochlorhydria) and intermittent. It plays an important part in the first digestive step, which if not completed properly can result in digestive problems all the way through the digestive tract. Some people with low stomach acid have no obviously related symptoms and are led to believe they are digesting properly, when they are not. An HCl trial is a safe, worthwhile, and relatively accurate way to discover if an insufficiency exists. HCl does not digest food on its own, but creates an environment in which digestion begins. HCl is responsible for converting pepsinogen to pepsin, which begins breaking down proteins in the stomach. With limited HCl, pepsinogen isn't converted to pepsin and protein digestion fails. A second action of HCl is to prevent infections, since most organisms that are ingested are destroyed by an adequate acid environment.

Since the long-term administration of supplemental HCl may result in reduced stomach output of acid, administration is advised for diagnostic purposes and short-term use only. Short-term use may result in a return of normal acid production.