For a thirsty person,drinking water serves to reduce,what? Instinct?

For a thirsty person, drinking water serves to reduce a drive.

A drive is an aroused state of psychological tension that typically arises from a need. A drive, such as hunger or thirst, motivates us to act in ways that will reduce the tension. So, for example, when you become thirsty (tension caused by need for water) you are motivated to drink (method of reducing the tension).

This will drive you to drink (or not)! Traces of prescription drugs have been found in far greater quantities in U.S. drinking water supplies than previously thought, a study has claimed. A report on drinking water carried out by the Environmental Protection Agency has found samples of at least 25 different drugs, including medication to treat heart conditions, in supplies coming out of wastewater treatment plants. Find out more here.