http://en.wikipedia.org/wiki/Bovine_somatotropin#Human_health
Milk from rBST-treated cows is chemically similar to traditional milk. Both have very similar levels of protein, fat, and sugar. A study also indicated that both sources of milk had comparable levels of vitamins and minerals. Milk from rBST-treated cows is not, however, chemically identical to traditional milk. Milk from rBST-treated cows contains slightly elevated levels of hormones such as BST, and IGF1. Cows treated with rBST have a substantially higher rate of mastitis and more often require antibiotics; however, strict regulations and testing prevent any milk tainted with antibiotics from being sold in most markets in the developed world
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BST is present in milk from both rBST-treated and untreated cows, but it is destroyed in the digestive system and even if directly injected, has no direct effect on humans
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The Food and Drug Administration,[34] World Health Organization,[4] the American Dietetic Association,[29] and National Institutes of Health[35] have independently stated that dairy products and meat from BST-treated cows are safe for human consumption. The American Cancer Society issued a report declaring “The evidence for potential harm to humans [from rBGH milk] is inconclusive. It is not clear that drinking milk produced using rBGH significantly increases IGF-1 levels in humans or adds to the risk of developing cancer. More research is needed to help better address these concerns.