Adverse effects

Common adverse reactions in patients taking opioids for pain relief include: nausea and vomiting, drowsiness, itching, dry mouth, miosis, and constipation.

Infrequent adverse reactions in patients taking opioids for pain relief include: dose-related respiratory depression (especially with more potent opioids), confusion, hallucinations, delirium, urticaria, hypothermia, bradycardia/tachycardia, orthostatic hypotension, dizziness, headache, urinary retention, ureteric or biliary spasm, muscle rigidity, myoclonus (with high doses), and flushing (due to histamine release, except fentanyl and remifentanil).

Opioid-induced hyperalgesia has been observed in some patients, whereby individuals using opioids to relieve pain may paradoxically experience more pain as a result of their medication. This phenomenon, although uncommon, is seen in some palliative care patients, most often when dose is escalated rapidly. If encountered, rotation between several different opioid analgesics may mitigate the development of hyperalgesia.

Both therapeutic and chronic use of opioids can compromise the function of the immune system. Opioids decrease the proliferation of macrophage progenitor cells and lymphocytes, and affect cell differentiation (Roy & Loh, 1996). Opioids may also inhibit leukocyte migration. However the relevance of this in the context of pain relief is not known.

Men who are taking moderate to high doses of an opioid analgesic long-term are likely to have subnormal testosterone levels, which can lead to osteoporosis and decreased muscle strength if left untreated. Therefore, total and free testosterone levels should be monitored in these patients; if levels are suboptimal, testosterone replacement therapy, preferably with patches or transdermal preparations, should be given. Also, prostate-specific antigen levels should be monitored.

Treating opioid adverse effects

Nausea: tolerance occurs within 7–10 days, during which antiemetics (e.g. low dose haloperidol 1.5–3 mg once at night) are very effective. Stronger antiemetics such as ondansetron or tropisetron may be indicated if nausea is severe or continues for an extended period, although these tend to be avoided due to their high cost unless nausea is really problematic. A cheaper alternative is dopamine antagonists, e.g. domperidone and metoclopramide. Domperidone does not cross the blood-brain barrier, so blocks opioid emetic action in the chemoreceptor trigger zone without adverse central anti-dopaminergic effects. Some antihistamines with anti-cholinergic properties (e.g. orphenadrine or diphenhydramine) may also be effective.

  • 5-HT3 antagonists (e.g. ondansetron)
  • Dopamine antagonists (e.g. domperidone)
  • Anti-cholinergic antihistamines (e.g. diphenhydramine)

Vomiting: this is due to gastric stasis (large volume vomiting, brief nausea relieved by vomiting, oesophageal reflux, epigastric fullness, early satiation), besides direct action on the vomiting centre of the brain. Vomiting can thus be prevented by prokinetic agents (e.g. domperidone or metoclopramide 10 mg every eight hours). If vomiting has already started, these drugs need to be administered by a non-oral route (e.g. subcutaneous for metoclopramide, rectally for domperidone).

  • Prokinetic agents (e.g. domperidone)
  • Anti-cholinergic agents (e.g. orphenadrine)

Drowsiness: tolerance usually develops over 5–7 days, but if troublesome, switching to an alternative opioid often helps. Certain opioids such as morphine and diamorphine (heroin) tend to be particularly sedating, while others such as oxycodone and meperidine (pethidine) tend to produce less sedation, but individual patients responses can vary markedly and some degree of trial and error may be needed to find the most suitable drug for a particular patient. Treatment is at any rate possible - CNS stimulants are generally effective.

  • Stimulants (e.g. caffeine, modafinil, amphetamine)

Itching: tends not to be a severe problem when opioids are used for pain relief, but if required then antihistamines are useful for counteracting itching. Non-sedating antihistamines such as fexofenadine are preferable so as to avoid increasing opioid induced drowsiness, although some sedating antihistamines such as orphenadrine may be helpful as they produce a synergistic analgesic effect which allows smaller doses of opioids to be used while still producing effective analgesia. For this reason some opioid/antihistamine combination products have been marketed, such as Meprozine (meperidine/promethazine) and Diconal (dipipanone/cyclizine), which may also have the added advantage of reducing nausea as well.

  • Antihistamines (e.g. fexofenadine)

Constipation: develops in 99% of patients on opioids and since tolerance to this problem does not develop, nearly all patients on opioids will need a laxative. Over 30 years experience in palliative care has shown that most opioid constipation can be successfully prevented: "Constipation ... is treated with laxatives and stool-softeners" (Burton 2004, 277). According to Abse, "It is very important to watch out for constipation, which can be severe" and "can be a very considerable complication" (Abse 1982, 129) if it is ignored. Peripherally acting opioid antagonists such as alvimopan (Entereg) and methylnaltrexone (Relistor) have been found to effectively relieve opioid induced constipation without affecting analgesia or triggering withdrawal symptoms. For mild cases, a lot of water (around 1.5 L/day) and fiber might suffice (in addition to the laxative and stool-softeners).

  • Stool-softening and peristalsis-promoting laxatives (e.g. docusate in combination with bisacodyl)
  • Peripherally-acting opioid antagonists (e.g. methylnaltrexone)
  • High water intake and dietary fiber

For more severe and/or chronic cases, the drugs that are used work by not increasing peristalsis, but by preventing water uptake in the intestine, leading to a softer stool with a larger component of water, and, additionally, by acidifying the environment inside the intestine, which both decreases water uptake and enhances peristalsis (e.g. lactulose, which is controversially noted as a possible probiotic). The following drugs are generally efficacious:

  • Polyethylene glycol 3350±10% dalton powder for solution (MiraLax, GlycoLax) 8.5-34g daily.
  • Lactulose syrup 10g/15mL 30-45mL twice daily.

Respiratory depression: although this is the most serious adverse reaction associated with opioid use it usually is seen with the use of a single, intravenous dose in an opioid-naive patient. In patients taking opioids regularly for pain relief, tolerance to respiratory depression occurs rapidly, so that it is not a clinical problem. Several drugs have been developed which can block respiratory depression completely even from high doses of potent opioids, without affecting analgesia, although the only respiratory stimulant currently approved for this purpose is doxapram, which has only limited efficacy in this application. Newer drugs such as BIMU-8 and CX-546 may however be much more effective.202122

  • Respiratory stimulants: carotid chemoreceptor agonists (e.g. doxapram), 5-HT4 agonists (e.g. BIMU8), d-opioid agonists (e.g. BW373U86)
  • Opioid antagonists (e.g. naloxone)

Finally, all opioid effects (adverse or otherwise) can readily be reversed with an opioid antagonist (more exactly, an inverse agonist) such as naloxone or naltrexone. These competitive antagonists bind to the opioid receptors with higher affinity than agonists but do not activate the receptors. This displaces the agonist, attenuating and/or reversing the agonist effects. However, the elimination half-life of naloxone can be shorter than that of the opioid itself, so repeat dosing or continuous infusion may be required, or a longer acting antagonist such as nalmefene may be used. In patients taking opioids regularly it is essential that the opioid is only partially reversed to avoid a severe and distressing reaction of waking in excruciating pain. This is achieved by not giving a full dose (e.g. naloxone 400 µg) but giving this in small doses (e.g. naloxone 40 µg) until the respiratory rate has improved. An infusion is then started to keep the reversal at that level, while maintaining pain relief.

See also