drinking a gallon and a half of water in a short time is dangerous and could lead to water intoxication. there was a girl who died recently from drinking 2 gallons of water in a short period of time in a radio contest to win a nintendo wii.
the SWF (Salt-Water-Flush) (salt water flush) im accustomed to involves drinking 32oz of water + 2 level teaspoon of Seasalt . sip it all down in one sitting. you may need to adjust the salt content according to your needs.