Phenformin is an anti-diabetic drug from the biguanide class. It was marketed as DBI by Ciba-Geigy but was withdrawn from most markets in the late 1970s due to a high risk of lactic acidosis, which was fatal in 50% of cases. Phenformin was discovered in 1957 by Ungar, Freedman and Seymour Shapiro, working for the US Vitamin Corporation. Clinical trials began in 1958 which showed it to be effective but with gastrointestinal side effects.