According to this story on msnbc.com about a trade dispute betwen the US and countries which ban the use of ractopamine:

Although few Americans outside of the livestock industry have ever heard of ractopamine, the feed additive is controversial. Fed to an estimated 60 to 80 percent of pigs in the United States, it has resulted in more reports of sickened or dead pigs than any other livestock drug on the market, an investigation of Food and Drug Administration records shows.

Growing concern over sick animals in the nation's food supply sparked a California law banning the sale and slaughter of livestock unable to walk, but that law was struck down by the Supreme Court Monday. Meat producers had sued to overturn California’s ban, arguing that the state could not supercede federal rules on meat production. The court agreed.

Since the drug was introduced, more than 218,000 pigs taking ractopamine were reported to have suffered adverse effects, as of March 2011, according to a review of FDA records. The drug has triggered more adverse reports in pigs than any other animal drug on the market. Pigs suffered from hyperactivity, trembling, broken limbs, inability to walk and death, according to FDA reports released under a Freedom of Information Act request. The FDA, however, says such data do not establish that the drug caused these effects.