When does a liver transplant become necessary?

When liver fails. When the degree of liver dysfunction does not permit the removal of toxins in the blood or the formation of proteins needed for normal survival like clotting proteins; the liver may need to be replaced. A failing liver can affect many vital functions including altering of mental status.
Symptoms are severe. Basically patients with chronic liver disease are eligible for placement on the waiting list when their symptoms and complications from liver failure and liver cirrhosis (portal hypertension) become severe and difficult to manage.