Two meta-analyses have been published on rBST's effects on bovine health.[6][7] Findings indicated an average increase in milk output ranging from 11%-16%, a nearly 25% increase in the risk of clinical mastitis, a 40% reduction in fertility and 55% increased risk of developing clinical signs of lameness. However, the study did not show if using "natural" growth hormones had the same or worse effects (the study in fact notes that synthetic BST can be made to be identical to natural BST). The same study reported a decrease in body condition score but speculated that it may have been attributable to differences in feeding of treated (underfed) versus untreated (overfed) cows.
A European Union scientific commission was asked to report on the incidence of mastitis and other disorders in dairy cows and on other aspects of the welfare of dairy cows.[8] The commission's statement, subsequently adopted by the European Union, stated that the use of rBST substantially increased health problems with cows, including foot problems, mastitis and injection site reactions, impinged on the welfare of the animals and caused reproductive disorders. The report concluded that, on the basis of the health and welfare of the animals, rBST should not be used. Health Canada prohibited the sale of rBST in 1999; the recommendations of external committees were that, despite not finding a significant health risk to humans, the drug presents a threat to animal health, and, for this reason, cannot be sold in Canada.[9]