Insulin

Insulin is used medically to treat some forms of diabetes mellitus. Patients with Type 1 diabetes mellitus depend on external insulin (most commonly injected subcutaneously) for their survival because the hormone is no longer produced internally. Patients with Type 2 diabetes mellitus are insulin resistant, have relatively low insulin production, or both; certain patients with Type 2 diabetes may eventually require insulin if other medications fail to control blood glucose levels adequately.

source: http://en.wikipedia.org/wiki/Insulin_therapy