Introduction to opioids

Opioids are strong analgesics used in the relief of pain, both acute and chronic, and cancer and non-cancer related. They are effective in nociceptive pain, and may be effective in neuropathic and inflammatory pain depending on the drug choice, dose and route. If used for long periods of time, opioids can be addictive. 

Mode of action

All opioid analgesics mimic endogenous (meaning produced by the human body) endorphins by stimulating opioid receptors in the central and peripheral nervous systems which results in relief of pain. Opioids are particularly useful in pain management as they:

  • Can be given by a variety of routes including oral, transmucosal, rectal, intravenous, subcutaneous, intramuscular, transdermal, epidural and intrathecal;
  • Are easily titrated to the correct dose;
  • Are highly effective;
  • Have a favourable risk benefit ratio.


Some reasons that people should not use opioids include:

  • Significant respiratory disease;
  • Comatose patients, unless near death;
  • Phaeochromocytoma (in some cases).

Adverse effects

Some adverse effects of opioids as a class include:


Opioids are drugs of addiction, however when they are used correctly their benefits outweigh this fact. When used in acute pain patients will not become addicted to them. When used for chronic pain, patients should be aware that they can be weaned off the drugs when the pain is no longer a problem.
As they are prone to causing constipation, opioids should be given with a laxative especially when required for regular analgesia.

Opioid analgesics

Weak opioids
Weak opioids are useful in mild to moderate pain. Codeine is the most commonly used weak opioid. Low doses of morphine, oxycodone or tramadol can also be used for mild to moderate pain.
Strong opioids
Strong opioids are used in moderate to severe pain. These include:

  • Morphine;
  • Hydromorphone;
  • Methadone;
  • Oxycodone;
  • Fentanyl;
  • Pethidine.

Other opioids

Buprenorphine is a partial opioid agonist which produces less effect than a strong opioid and exhibits a ceiling effect, after which increasing doses will not produce additional analgesia. When given in addition to a full agonist, it may displace the full agonist (eg. morphine), which can precipitate pain and opioid withdrawal syndrome. Tramadol is a synthetic codeine analogue that is effective for mild to moderate pain. It has a dual mechanism of action: it is an opioid agonist, and it inhibits presynaptic uptake of noradrenaline and serotonin. It appears to be more effective than other opioids for neuropathic pain and its effects are only partially reversed by naloxone.

For more information about opioids for analgesia, talk to your doctor.


  1. Australian Medicines Handbook. ed. 2006. Australian Medicines Handbook Pty Ltd.
  2. Woodruff, R. Cancer Pain. 2nd ed. 1999. Mundipharma Pty Ltd.

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