Recombinant bovine growth hormone (rBGH), is a genetically engineered growth hormone that's commonly injected into dairy cows. rBGH forces dairy cows to artificially increase milk production by about 10% to 15% but sometimes can increase milk production by as much as 40%.
The FDA approved rBGH in 1993, but various advocates for safe food feel that rBGH was never properly studied for negative health effects. Current research shows that rBGH can cause harm, many time over to cows and also poses various health risks for humans.
See a map of dairy products that do not contain rBGH.