MATH PUZZLE: Can you replace...

A man went to his doctor and told him that he had not been feeling well lately. The doctor examined the man, left the room, and came back with three different bottles of pills. “Here take the green pill with a big glass of water when you wake up,” he said. “Take the blue pill with a big glass of water after you eat lunch. Then just before going to bed take the red pill with another big glass of water.”
Worried to be put on so much medicine the man said. “Oh, Doc! Now exactly what is my problem?”
The doctor replied, “You are not drinking enough water.”