No side effects of bovine growth hormone in humans have yet been scientifically proven, according to the FDA. Recombinant bovine growth hormone is a synthetic version of a growth hormone that is injected into dairy cows in the United States to increase milk production.
When rBGH is injected into a cow, it in turn increases the cow's level of insulin-like growth hormone. These hormones transfer into the cow milk and then to humans who consume it. Higher amounts of IGF-1 have been linked to the development of certain cancers, but research on causation is inconclusive.
rBGH has been permitted by the FDA since 1993. It is not allowed in Canada or the European Union.