My mom had gone to one of those diet clinics allegedly run by a doctor and they give you shots everyday. The tell you to drink one oz of water for every lb of body weight (that is about 1 1/3 gallons for her and she is about 5' tall) She was feeling dizzy and called them and they said to just keep drinking more water, more water. She did and the symptoms got worse. She went to her regular doctor and upon testing her electrolytes were so depleted that she was a few numbers short of going into a coma.
That is just her experience but the regular physician said it was from too much water.