Chronic pain – the kind that lasts for months or recurs regularly – afflicts more than a quarter of adult Americans. Treating pain can be extremely challenging, however, in part because it can't be measured with instruments. It's in the eye – or neck or joint – of the beholder.

Doctors often prescribe powerful painkillers called opioids -- natural or synthetic versions of opium. Sometimes the prescription is for short-term, acute pain: If you've ever had a root canal or surgery or thrown out your back, you may have received a prescription for Percocet or Vicodin, both of which are opioids that also contain acetaminophen.

For people with long-term, persistent pain – often from musculoskeletal injuries or nerve damage – opioids may be the best option to manage their pain and enable them to function day after day.

But there's a hitch: Though highly effective, these drugs are dangerous and addictive. The chief danger is that they can cause respiratory depression: If too much is taken, breathing slows and may eventually stop.

And because they cause euphoria, opioids are popular targets for misuse and abuse. In 2007, 11,499 people in the United States died from opioid overdoses, according to the Centers for Disease Control and Prevention. That was more than the number of overdose deaths for heroin and cocaine combined.

To help monitor use of the drugs, some doctors ask patients to sign "pain contracts" or "opioid treatment agreements" that spell out the rules patients must follow to take these drugs safely. The contracts aim to discourage people from taking too much medication, mixing medications, or sharing or selling them, among other things.

The agreements may require patients to submit to blood or urine drug tests, fill their prescriptions at a single pharmacy or refuse to accept pain medication from any other doctor. If patients don't follow the rules, the agreements often state that doctors may drop them from their practice.

Some patient advocates and policy experts say that rather than ensuring safety, the agreements invade patients' privacy and damage the trust that's essential to the doctor-patient relationship.

Joan Crowley started taking an opioid in 2003 to treat recurring migraines and an arthritis-like autoimmune disorder that caused her joints to swell. The drug kept her pain under control and allowed her to continue her work as an accountant in the Pittsburgh area.

A few years ago, her primary-care provider asked her to sign a treatment agreement for the opioid and for Xanax, an anti-anxiety drug she also took regularly. Every three months she visited the doctor so he could evaluate her condition and write her a new round of prescriptions. Sometimes he did a urine test as well.

All went smoothly until this past winter. Crowley, 51, went to the emergency room with what she thought was a heart attack but turned out to be anxiety. While there, she says, she was given an anti-anxiety drug and other medications.

The next day she had her regular appointment with her doctor, who gave her prescriptions for her regular drugs and took a urine sample. A week later, she says, she got a telephone call from him, saying that an opioid she wasn't supposed to be taking had turned up in her urine sample. The doctor gave her 60 days to find a new physician – even after she told him about her ER visit.

Crowley acknowledges that her relationship with the doctor had been strained before that. Still, she was stunned. "This is someone I'd been a patient of for 11 years," she says. "There was a level of trust there."

Because of a few high-profile prosecutions of doctors for running "pill mills," some experts say, doctors increasingly use pain contracts to protect themselves. The subjective nature of pain makes doctors afraid they'll be scammed by unscrupulous patients, says Myra Christopher, chief executive of the Center for Practical Bioethics in Kansas City, Mo., who co-authored a recent article critical of pain contracts.

"Providers' primary concern ought to be the management of pain and suffering," she says. "This shifts the locus of concern to the providers' protection."

Others disagree. They say treatment agreements can function as an educational tool and a treatment road map. "It provides a framework to talk about the issues that come up in a treatment plan," says S. Hughes Melton, a family physician in rural Lebanon, Va., where substance abuse, including addiction to pain medication, is a serious problem.

After working in the mining industry for 22 years, Jeffery Boyd, 50, developed continual pain in his back and legs. Working with Melton, he manages his pain with an opioid and another drug. To Boyd, signing a treatment agreement and being closely monitored by Melton are secondary concerns: Mostly he's just glad to have his pain under control.

"The pain won't ever go away," he says, "but [Melton] got me to where I can work at my job and do things."

That attitude is probably shared by many people with chronic pain, say experts. "Most patients who come in, they just want relief," says Will Rowe, chief executive of the American Pain Foundation, a consumer advocacy group. "They don't want to hear about the public-health problem of the misuse of opioids."