8th January 2024

Side effects

Strong and weak opioids cause many side effects. The most important things to be aware of for the exam are discussed here, but this is by no means an exhaustive list.

Constipation

Constipation is a significant side effect, affecting 60% of patients taking opioids. It is the most frequently reported side effect. NICE guidelines recommend commencing laxatives alongside opioids: senna is recommended as a first choice, at 15mg at night with dose titration and addition of other agents based on response.

Mu receptors in the GI nervous system are bound by opioids, causing slowed GI transit, increased water reabsorption, and constipation. This may lead to cramping abdominal pain from an increase in non-propulsive contractions.

Nausea

Opioid-induced nausea is less common than constipation but is still a side effect to recognise and manage. Nausea is a common transient side effect shortly after commencing opioids. Delayed gastric emptying and direct effects on the chemoreceptor trigger zone are two mechanisms where symptoms continue long-term.

Antiemetic therapy, in this case, depends on history:

• In the context of associated bowel dysfunction, constipation should be well managed. A prokinetic antiemetic such as metoclopramide 10mg three times a day would be the first choice.
• If there were no bowel symptoms, a centrally-acting antiemetic such as haloperidol is preferred. Ondansetron could be an option, but this is frequently constipating, so may worsen symptoms.

Respiratory depression

This is a life-threatening, easily reversible emergency. Risk factors for developing respiratory depression include rapid increase in opioid dose, deterioration in renal function, and co-prescription of benzodiazepines and gabapentinoids. Rapid treatment with Naloxone is required to reverse the effect of the opioid.

Naloxone administration can cause a severe acute withdrawal syndrome and pain. The dose must be carefully titrated against the respiratory rate:

• Where the respiratory rate is >8, monitor only. Reduce or remove any opioids.
• Where the respiratory rate is <8, and the patient is arousable, give 100 micrograms IV Naloxone every 2 minutes until respiratory rate >8.
• Where there is no respiratory effort, 400 micrograms IV Naloxone should be given, followed by 800 micrograms after 1 minute if no response.