Who wouldn't want to be red carpet perfect? The pressure to be thin screams from headlines everywhere. And imagine the pressure for people who work in the public eye. "Jennifer" is a model who prefers to remain anonymous. But the 20-year-old says there was a time that she would do anything to fit the Hollywood mold of perfection...even take a diet pill that she knew could kill her. That pill is called Clenbuterol. It is also known as "The Size Zero Pill." Clenbuterol is normally used to treat asthma in horses. But the drug also causes rapid weight loss in humans. Dr. Shailesh Patel, an endocrinologist from the Medical College of Wisconsin, calls it a dirty little weight loss secret that first made the rounds among Hollywood actresses. "When you see a nice young actress who's clearly been a little plump a few months before, and all of a sudden a few weeks later you see her being a little gaunt, she almost looks like she's been on a severe diet. There's no doubt in my mind that it's more than a severe diet." But Jennifer says the lure of the Size O Pill is very strong. "My body would look better than it already did, and what 20-year-old wouldn't be like, 'Alright! I want my body to look better than it does.'" This is how Clenbuterol works: It mimics adrenaline in the human body, boosting your metabolism. It raises your heart rate, and mobilizes fat from your fat stores so you have energy. The result is rapid weight loss, and you don't even have to diet or exercise. TODAY'S TMJ4 reporter Shelley Walcott found Clenbuterol for sale on dozens of Web sites, no prescription or proof of age required. But those pushing the drug online rarely mention the dangerous effects it can have on your heart or central nervous system. It's something Jennifer felt firsthand. "The most prevalent thing was the rapid heartbeat. Just like my heart was going to beat out of my skin. This wasn't a cup of coffee feeling. This was like a three pots of coffee feeling. Just out of nowhere my eye would just start going crazy. Twitching. Just from my nerves being so out of control," she explains. Dr. Patel says those symptoms can be deadly. He says Clenbuterol makes your blood pressure and heart rate soar, raising the risk of fatal arrhythmias and stroke. And get this: The body stops responding to Clenbuterol after just a few months. "It might just get you into a bikini or your favorite dress," Patel says, "but it's all going to come right back." Jennifer says she has stopped using Clenbuterol, and she says she's learned her lesson. "There is nothing at all that exists that's going to take the place of a consistent, good nutritional plan and working out... exercising on a regular basis, nothing." Clenbuterol is not licensed by anyone for use in humans. It is illegal to use, and could be deadly.