Peptides are generally considered to be poor drug candidates because of their low oral bioavailability and propensity to be rapidly metabolized. The concept that a drug can be not ‘orally available’ has become more and more accepted and, as a consequence, some pharmaceutical companies have contributed in recent years to a revival of interest in peptides as potential drug candidates. New synthetic strategies to improve productivity and reduce metabolism of peptides, along with alternative routes of administration, have been developed in recent years, and a large number of peptide-based drugs are now being marketed.