Why we should not use more chlorine tablets than specified????

 
Chlorine reacts with naturally occurring organic matter to form trihalomethanes(THM) and halogenated acetic acids (HAA). High concentrations of these have detrimental health effects. They are carcinogenic and therefore chlorine can be added to drinking water for disinfection only upto a certain concentration.
The WHO drinking water standards state that 2-3 mg/L chlorine should be added to water in order to gain a satisfactory disinfection and residual concentration. The maximum amount of chlorine one can use is 5 mg/L.