When you drink too much water, you may experience water poisoning, intoxication, or a disruption of brain function. This happens when there is too much water in the cells (including brain cells), causing them to swell. When the cells in the brain swell they cause pressure in the brain. You may start experiencing things like confusion, drowsiness, and headaches. If this pressure increases it could cause conditions like hypertension (High Blood Pressure) and bradycardia (Low Heart Rate).