Complex sentence: The most common treatment, as it is cheap and widely available, is a special clear nail polish that, once applied to the nails, releases a bitter flavor on contact with the mouth which discourages the habit and has demonstrated its effectiveness.
Split into simpler sentences: The most common treatment, as it is cheap and widely available, is a special clear nail polish that has to be applied to the nails. It releases a bitter flavor on contact with the mouth which discourages the habit and has demonstrated its effectiveness.