Don't take an opioid painkiller to treat chronic pain until you have tried other, less risky, pain relievers - such as acetaminophen, a non-steroidal anti-inflammatory drug (NSAID), or salsalate.

Some kinds of chronic pain in particular - such as nerve pain, migraines, or fibromyalgia - are best treated with other types of drugs, not opioids. Talk with your doctor about non-drug measures, too. Studies show they can ease chronic pain, either alone or in combination with drugs. These include cognitive behavioral therapy, exercise, spinal manipulation, and physical rehab programs.

The weight of medical evidence indicates that while opioids are highly effective - and usually the drugs of choice - in relieving acute severe pain, they are only moderately effective in treating long-term chronic pain, and their effectiveness can diminish over time.

In addition, while the long-term use of opioids has not been well studied, it has been linked to: (1) a decrease in sex hormones leading to both a loss of interest in sex and impaired sexual function; (2) a decline in immune function; and (3) an increase the body's sensitivity to pain.

Opioids also can be addictive and are prone to be abused and misused, though they are rarely addictive in people who genuinely need them to control pain.

 If other options fail, your doctor may consider an opioid since controlling pain is always a medical priority. Taking effectiveness, safety and side effects, dosing flexibility and convenience, and cost into account, we have chosen the following opioids as Best Buys for people with moderate to severe chronic pain when other pain relievers fail to bring adequate relief:

• Generic codeine plus acetaminophen
• Generic morphine extended release
• Generic oxycodone extended release
• Generic oxycodone with acetaminophen

These four medicines have a long track record, provide good value. They range widely in monthly cost, depending on dosing regimen. But most low-dose regimens will run you less than $150 a month or so.

High doses and some extended release formulations of these medicines can be quite expensive. If you need to take high doses, we advise speaking with your doctor about which opioid has the lowest cost under your insurance plan. If you have to pay out of pocket, take care to avoid the high-cost versions of our Best Buys.

There is no reason to take the brand-name versions of our Best Buys, or any opioid for that matter. That includes well-known brands such as OxyContin, Percocet, and Vicodin.

*Selected doses. For the complete list of prices for all dose strengths, see our full report.