Referred to as the happy chemical, serotonin (5-hydroxytryptamin) is known to contribute to overall happiness and wellbeing—and that’s why many choose to take natural serotonin antagonists. A person suffering from major depression, for example, likely has decreased serotonin levels and activity, which results in low energy, loss of interest in daily activities, and low self-esteem. Even IBS patients tend to worry about the chemical. Constipation and low serotonin are linked, which is made worse by reduced serotonin sensitivity in the rectal region.