Clenbuterol is a substituted phenylaminoethanol that has beta-2 adrenomimetic properties at very low doses.  Unusually for a adrenergic beta-agonist, it has some anabolic effects, possibly redirect metabolism from fat to muscle.
The drug is approved for veterinary use as a bronchodilator in horses, and is also used to prevent fat gain in pigs. In humans, it is used as a weight loss drug, but is not widely used due to toxicity. "When the drug is used in animals and humans ingest liver or muscle containing the drug, they may experience an increase in heart rate, nervousness, headache, muscular tremor, dizziness, nausea, vomiting, fever, and chills, typically resolving within 2 to 6 days. As a result of such consequences in humans, the United States and Europe actively monitor urine and tissue samples from livestock for the presence of clenbuterol."
In February 2009, 70 people in China have become ill due to clenbuterol consumed in contaminated pork.