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Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for opioid use disorder, reversing opioid overdose, suppressing cough, suppressing opioid induced constipation, as well as for executions in the United States. Extremely potent opioids such as carfentanil are only approved for veterinary use. Opioids are also frequently used non-medically for their euphoric effects or to prevent withdrawal.

Opioids are effective for the treatment of acute pain (such as pain following surgery). For immediate relief of moderate to severe acute pain opioids are frequently the treatment of choice due to their rapid onset, efficacy and reduced risk of dependence. However a new report showed a clear risk of prolonged opioid use when opioid analgesics are initiated for an acute pain management following surgery or trauma. They have also been found to be important in palliative care to help with the severe, chronic, disabling pain that may occur in some terminal conditions such as cancer, and degenerative conditions such as rheumatoid arthritis. In many cases opioids are a successful long-term care strategy for those with chronic cancer pain.

Tolerance is a process characterized by neuroadaptations that result in reduced drug effects. While receptor upregulation may often play an important role other mechanisms are also known. Tolerance is more pronounced for some effects than for others; tolerance occurs slowly to the effects on mood, itching, urinary retention, and respiratory depression, but occurs more quickly to the analgesia and other physical side effects. However, tolerance does not develop to constipation or miosis (the constriction of the pupil of the eye to less than or equal to two millimeters). This idea has been challenged, however, with some authors arguing that tolerance does develop to miosis.

Tamper-release formulations of time-controlled preparations of medications are intended to curb abuse and addiction rates while trying to still provide legitimate pain relief and ease of use to pain patients. Questions remain, however, about the efficacy and safety of these types of preparations. Further tamper resistant medications are currently under consideration with trials for market approval by the FDA.

Opioid-induced constipation (OIC) develops in 90 to 95% of people taking opioids long-term. Since tolerance to this problem does not develop readily, most people on long-term opioids need to take a laxative or enemas. While all opioids cause constipation, there are some differences between drugs, with studies suggesting tramadol, tapentadol, methadone and fentanyl may cause relatively less constipation, while with codeine, morphine, oxycodone or hydromorphone constipation may be comparatively more severe. Opioid rotation is commonly used to try and minimise the impact of constipation in long-term users.

Persons performing any safety-sensitive task should not use opioids. Health care providers should not recommend that workers who drive or use heavy equipment including cranes or forklifts treat chronic or acute pain with opioids. Workplaces which manage workers who perform safety-sensitive operations should assign workers to less sensitive duties for so long as those workers are treated by their physician with opioids.

Opium became a major colonial commodity, moving legally and illegally through trade networks involving India, the Portuguese, the Dutch, the British and China, among others. The British East India Company saw the opium trade as an investment opportunity in 1683 AD. In 1773 the Governor of Bengal established a monopoly on the production of Bengal opium, on behalf of the East India Company. The cultivation and manufacture of Indian opium was further centralized and controlled through a series of acts, between 1797 and 1949. The British balanced an economic deficit from the importation of Chinese tea by selling Indian opium which was smuggled into China in defiance of Chinese government bans. This led to the First (18391842) and Second Opium Wars (18561860) between China and Britain.

Drug misuse and non-medical use include the use of drugs for reasons or at doses other than prescribed. Opioid misuse can also include providing medications to persons for whom it was not prescribed. Such diversion may be treated as crimes, punishable by imprisonment in many countries. In 2014, almost 2 million Americans abused or were dependent on prescription opioids.