Not drinking enough water

One afternoon, a man went to his doctor and told him that he hadn't been keeping well lately. The doctor examined the man, left the room and came back with three different bottles of pills. 

Then he gave the patient the following instructions.

Take the green pill with a big glass of water when you wake up. 
Take the blue pill with a big glass of water after lunch. 
Take the red pill with big glass of water just before going to bed'.

Startled at being put on so many medicines, the man asked, 'Doctor, what exactly is the problem with me?'.

The doctor replied calmly , 'You're not drinking enough water.'