This article looks at opioid therapy. Opioids are a class of compounds derived from the opium poppy. Extract of opium has been used since prehistory for pain relief and herbalists even today will make a tea from dried poppy pods to help with moderate pain. In modern times, the compounds have been purified and sole separately, usually based on how well they block pain and their side effects. The most common are codeine and its derivatives and morphine. In the illegal market, heroin is also an opioid.
Along with the scale of potency, opioids also have side effects and addictive properties. In general, the stronger a drug is against pain the greater the risk of side effects and addiction. For this reason, medical personnel will try to match the lowest strength product that relieves pain for opioid therapy. The idea is to gain the pain relief benefits with the lowest risk of adverse reactions.
In chronic pain, there are other considerations with opioid therapy. Pain that is acute and short-lived can be treated with any opioid that gives relief without much risk of addiction or long-term problems. The main concern with opioid therapy would be allergies, nausea or constipation. At high doses, breathing suppression is seen, but this is rare in doses prescribed for outpatient use. When these same drugs in opioid therapy are taken over weeks, months or even years, tolerance develops to the side effects and patients who are not allergic will have minimal problems with gastrointestinal upset or constipation.
The three main worries with chronic use of opioid therapy are hormonal changes, immune system changes and hyperalgesia (sensitivity to pain). Hormonal changes can cause decreased testosterone and cortisol, leading to loss of libido and gynocomastia in men and amenorrhea in women. The immune system, when compromised, makes infections more likely and longer lasting when they occur. Hyperalgesia is an effect where chronic opioid users become more sensitive to pain generally, not just to the pain from the original problem.
The hyperalgesia is a tricky problem. Because tolerance and disease progression can mean a patient needs a higher dose of opioid, when hyperalgesia occurs, physicians may not be able to distinguish it from a tolerance or a worsening of the underlying condition. Increasing the dose in opioid therapy after a patient is stable is risky as well. One option is to try switching to another type of opioid, although this too has some risks.
Generally, opioid therapy for chronic pain is both safe and effective, as long as a physician is monitoring the patient, especially during the first weeks and months while they become stabilized. A review from 2003 on opioids can be found here (New England Journal of Medicine).