Many of us are told from a young age not to swallow toothpaste. Toothpaste contains lots of fluoride, and while this mineral strengthens enamel and protects teeth from decay, in large quantities it is poisonous. However, you'd need to swallow a whole tube in one go to experience ill-effects. Actually dentists advise adults not to rinse their mouths with water after brushing teeth as this washes the flouride away. You also don't need to wet the toothbrush before hand.