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Opioids (narcotic analgesics) are a class of medicines that are used to provide relief from moderate-to-severe acute or chronic pain. They may also be called opiates, opioid analgesics, or narcotics. Analgesic is another name for a medicine that relieves pain. An opioid analgesic is an opioid that can be prescribed by a doctor to relieve pain.

Opioid analgesics are one of the most widely used analgesics for pain relief; however, they have been overused, overprescribed, and misused which has resulted in more than two million people in the U.S. alone having a substance misuse disorder involving prescription opioid analgesics.

Opioids work by binding to opioid receptors, which are part of the messenger system in our body that controls pain, and pleasurable and addictive behaviors. Opioid receptors are more abundant in the brain and spinal cord but are also located elsewhere in the body such as the stomach and the lungs. The main opioid receptor that opioids bind to is the mu receptor.

What are opioid analgesics used for?

In the past, opioid analgesics have been used for all types of pain, which may explain why there are so many people addicted to them today.

The most appropriate use of opioid analgesics is for the relief of short-term, intense pain, such as that occurring immediately after surgery or due to a medical condition.

Opioid analgesics may also be used to relieve pain due to cancer, or for palliative or end-of life care. They should only be used to treat other types of chronic pain under strict conditions and with close monitoring.

What are the differences between opioid analgesics?

Opioid analgesics differ in their structure, potency, and the way they are absorbed, distributed, metabolized and excreted within the body.

Some, like morphine and codeine were originally derived from plants. Others, such as heroin, hydrocodone, hydromorphone, oxycodone, and oxymorphone are made by modifying morphine and are called semi-synthetic. There are three main classes of opioids – those that are structurally like morphine (the phenanthrenes), those that resemble fentanyl (the phenylpiperidines), and those that resemble methadone (the phenylheptylamines).

Different opioid analgesics have different potencies, based on how strongly they bind to the opioid receptor (for example, fentanyl is 80 to 100 times stronger than morphine). This means that the dosages for one opioid may be significantly different from another. Although conversion charts exist (these tell you what dose of an opioid compares to the equivalent analgesic dose of morphine); these are only at best a guide because other variables, such as an individual’s genetics, play a part in how a person responds to a opioid.

If an opioid analgesic is deemed appropriate; codeine or tramadol should be tried first if the pain is mild-to-moderate. If the pain is unresponsive to these analgesics, or for more severe pain, then hydromorphone, morphine, or oxycodone should be considered. Fentanyl and methadone should only be used for severe pain that is unresponsive to other opioid analgesics.

Common opioid analgesics available in the U.S. include:

Are opioid analgesics safe?

There are serious risks associated with opioid analgesics including respiratory depression (unusually slow and shallow breathing), opioid-use disorder, and potentially fatal overdose.

Opioid-use disorder is defined as the repeated occurrence of at least two out of 11 specific opioid-associated problems, including using opioids in increased amounts or for longer than intended; continued use despite interference with daily activities; or still using while in hazardous situations. Severe opioid-use disorder is defined as a cluster of 6 or more problems and it can affect people from all educational and socioeconomic backgrounds.

Opioid analgesics are potentially addictive and the risk of becoming emotionally and physically dependent on them increases the more you take and the longer you take them for. When prescribed by a doctor and used for short periods of time, such as for less than five days for pain relief after surgery, the risk of becoming addicted to opioid analgesics is relatively low. The number of fatal overdoses from opioids continues to rise. 

When taken exactly as prescribed by a doctor, for short periods of time, for the relief of severe pain, and under strict monitoring, opioid analgesics are considered acceptable to use. However, they may still be associated with some severe side effects, despite proper use.

What are the side effects of opioid analgesics?

Opioid analgesics have many side effects, although people with cancer or terminal illness taking opioids for long periods of time may become tolerant to some of these side effects.

Drowsiness, sleepiness, or dizziness is common with most opioid analgesics. This can affect driving or a person's ability to operate machinery and perform other hazardous tasks. Alcohol may potentiate these effects.

Other commonly reported side effects include:

• Constipation
• Breathing difficulty
• Itching or a skin rash
• Loss of appetite
• Low blood pressure
• Miosis (constriction of the pupil of the eye)
• Nausea and vomiting
• Problems with urination
• Shakiness or a tremor
• Slow heartbeat.

Withdrawal symptoms may occur when people who have been taking opioid analgesics abruptly stop taking them. Symptoms are like having the flu, and may include body aches, chills, depression, diarrhea, goose-bumps, headaches, high blood pressure, insomnia, irritability, a runny nose and sweats. Withdrawal symptoms usually last about a week.