thanks for your query at HCM listerine
is quite safe, mouth washes are generally made safe because of chance of accidental swallowing are more.
Ingestion of large quantities may cause may cause headache, dizziness
nausea, vomiting, diarrhea, dehydration
, and symptoms of drunkenness
large quantity in here means taking almost 2- 4 gram per kg , that would be very high quantities.
small amount will not cause any significant side effects
hope this is helpful