Ractopamine is a steroid originally designed to treat human asthma, but it's been found to increase the growth rate of some pigs. Unfortunately it also causes heart failure in many of them, though slaughterhouses just chop those pigs into pork early and off it goes to you.
Banned in 160 Nations, Why is Ractopamine in U.S. Pork?
This has become more awkward for the pork industry given that 160 countries have banned pork treated with the drug, and so, banned US pork. The pork industry is trying to force Europe to accept its drugged pork, and Europeans are protesting.
US pork producers' use of drug may derail European trade deal
The pork industry, for its part, says that Europeans aren't listening to the science - except the only human safety test of Ractopamine involved 6 men, one of whom had to drop out because his heart began racing erratically. There's no evidence to show it's safe in any dose for humans, and given how similar pigs are to us and their pattern of heart failure with the drug, it's probably not wise for us to be eating it.