Jan. 4, 2008  -- After years of reoccurring kidney stones and regular surgeries, Jared Hess became addicted to painkillers.

After a monthlong stay in the hospital and being prescribed the powerful painkiller Oxycontin, Hess continued to use the drug against his doctors' orders, surreptitiously obtaining pills.

Within a year of first being prescribed the painkiller, he found himself in rehab. He was just 19 years old.

"I was in college when it first became a real problem. I lost interest in school, stopped going to class, my relationships with friends and family deteriorated," Hess, now 24, told ABCNEWS.com.

"I was doing it every day and by myself. My life revolved around getting the drug and using it," he said.

Hess now works for the nonprofit Faces and Voices of Recovery, which advocates for substance abusers, who like Hess, often have a hard time getting insurance companies to pay for their treatment.

Hess embodies the two groups experts say are most susceptible to painkiller abuse — patients prescribed drugs who later become addicted, and young people who generally begin using the drugs recreationally and not for medical reasons.

An estimated 5.2 million people used prescription pain relievers in 2006 for nonmedical reasons, up from 4.7 million in 2005, according to the Department of Health and Human Services. That's more than twice the 2.4 million people the department estimates use cocaine nationwide.

According to statistics compiled by the Partnership for a Drug Free America, nearly one in five teens, or a staggering 4.5 million kids age 12-19, reportedly abused prescription medications to get high last year. Despite an overall downward trend in overall drug use among teenagers, painkiller abuse is up, according to a White House report released by President Bush last month.

What makes opioids — the class of common pain drugs like Oxycontin and Vicodin — effective pain relievers is also what makes them so highly addictive, said Fred Berger, medical director of the Scripps McDonald Center, the drug rehabilitation center at Scripps Memorial Hospital La Jolla, Calif.

"Opioids are chemicals that attach to certain receptors in the brain. … These drugs both prevent pain and stimulate the pleasure center in the brain. Those drugs that are the most effective in terms of attaching to those receptors give the most relief from pain as well as the most pleasure."

Berger described a wide range of people who become addicted to painkillers from "little old ladies who fractured a vertebra, are placed on meds and then can't come off them or don't want to," to "teenagers looking through their parents' medicine cabinets in order to get high."

The most common medical problem abusers who get hooked typically have is lower back pain, he said.

"These drugs serve a purpose and that's to deal with short-term pain. There are physicians who prescribe drugs chronically and after a while patients become habituated. Patients need more medicine to have an effect, but the pain doesn't get any better. They become dependent and if they try to stop withdrawal symptoms set in and the pain becomes more accentuated," he said.

Of those patients legitimately prescribed painkillers, people with addictive personalities or who have been addicted to other substances in the past are particularly prone to developing addictions, experts told ABC News.

"Some people are more prone to addiction," said Joseph A. Califano Jr., chairman and president of the National Center on Addiction and Substance Abuse at Columbia University. "People who are depressed, prone to anxiety or alcoholics are more likely to develop an addiction to prescription drugs."

Doctors, he said, have to know their patients' histories with substance abuse and remain involved in their treatment while taking potentially addictive painkillers.

"Part of any physician's responsibility is to carefully watch their patients. … It is vital they question their patients enough to determine if they might become addicted. If a doctor focuses on an alcoholic's pain, or anxiety, or sleeplessness, and ignores his alcoholism, there is an increased chance of developing a prescription drug addiction," he said.

Unlike Hess, the majority of teenagers who become addicted to painkillers were never prescribed medication by a doctor. These kids instead find the drugs in their parents' medicine cabinets, get them off friends or purchase them off the Internet.

"There has been an explosion of prescription drug abuse among teens," Califano said. "They see their parents using these drugs and they think they are using a clean pill approved by the FDA [Food and Drug Administration]. They think the drugs are safe because they're not buying it from dirty drug dealers on dirty corners. The increase in prescription drug abuse has wiped out any of the modest decreases among teen users of marijuana."

Teenagers looking to get high often crush and snort pills, quickly releasing the drug intended to be spread throughout the body during 12 to 24 hours. Users report a high similar to other opiates like heroin that can "really affect their functioning," said Berger.

The visible effects on a typical painkiller abuser, however, are subtle and family members of addicts sometimes rarely know.

"There is very little medical damage that goes along with opioid addiction," Berger said. "It's destructive because it gets you by the throat in terms of addiction potential. Your life becomes consumed with worrying about how to get the medication. It is an ongoing process centered on getting drugs, maintaining your supply and worrying little about the other important aspects of your life."