The Prescription

A guy goes to his doctor. After the doc examines the patient, he takes out three bottles from his desk, hands them to the patient and says:

"Right, you take one green pill first thing in the morning with lots of water. You take one of the yellow pills after lunch with lots of water. Lastly, you take one of the white pills before bed, again with lots of water." The guy gets scared. "Doc, what's wrong with me?" "You're dehydrated."