A hormone used to increase milk yield in cows - recombinant bovine somatotropin (rbST) - is proving to be one of the most controversial drugs of the decade. Since the 1980s there have been fears over the safety of milk from rbST-treated cows, and last year, a leaked internal report by Health Canada scientists identified numerous gaps in the safety data on human and animal safety. Last week, Health Canada announced that, unlike many other agencies, it would not approve the drug. US consumer groups now want the drug withdrawn.