Swallowing small quantities of Listerine does not cause any harm, but it should be avoided. However, swallowing large amounts may lead to mouthwash overdose and requires immediate medical attention.
Swallowing large quantities of Listerine is similar to alcohol intoxication. Mouthwash overdose occurs when someone swallows a large amount of mouthwash liquid intentionally or accidentally. Some of the symptoms of mouthwash overdose include dizziness, abdominal pain, low blood pressure, headache, drowsiness, rapid heart rate, throat pain, low blood sugar and uncoordinated movements, as noted by MedlinePlus.
Those who have any concern of overdose after swallowing Listerine should immediately contact the National Poison Control Center at (800) 222-1222 for professional help, or go to the emergency room to receive medical attention. Severe cases may lead to admittance at the hospital.