Opioid-induced constipation

The gut has its own network of nerves with opioid receptors. When an opioid binds to them, the coordinated squeezing of the bowel wall slows, fluid is absorbed more fully from the stool and the muscle that controls the back passage can tighten. The stool becomes harder, drier and harder to pass.

Many side effects such as nausea and sleepiness lessen over days or weeks. Constipation is different: people generally stay prone to it for as long as the opioid is taken, which is why a routine rather than an occasional remedy is the norm for regular users.

Infrequent stools are the usual sign, but people also describe straining, hard pellets, a feeling of incomplete emptying, bloating, cramps, loss of appetite and nausea. In older or frail people, confusion or sudden restlessness can sometimes be the first clue that the bowel is impacted.

The NHS notes that long-term constipation can lead to faecal impaction, where hard stool builds up in the rectum and liquid stool may leak around it, which looks like diarrhoea but is not. Treatment may then need stronger prescription laxatives, suppositories, mini-enemas or manual removal by a healthcare professional.

Practical habits help, even though they rarely solve opioid constipation alone. The NHS suggests increasing fibre gradually, drinking enough fluid, staying as active as possible and keeping a regular toilet routine, going when the urge arises rather than delaying. Fibre without enough fluid can make things worse, so adjust both together.

• Ask the prescriber at the start whether a laxative should begin at the same time as the opioid.

• Try to use the toilet at a consistent time, often after a meal.

• Raise the knees with a small footstool when sitting.

• Keep a simple note of bowel movements so changes are obvious.

• Tell the pharmacist about all other medicines, since several others also constipate.

For opioid constipation, clinicians often favour a stimulant, an osmotic laxative or a combination, because bulk-forming products rely on a bowel that can still move. The NHS says to check with a pharmacist or doctor before taking laxatives if you are on opioid painkillers such as codeine or morphine, so that the choice fits your situation. Most laxatives work within about 3 days and are meant for short-term use unless a prescriber has agreed a longer plan.

If constipation persists despite a well-chosen routine, the GP or specialist team can review the opioid itself. Options may include changing the opioid, adjusting how it is taken, looking for other contributors such as low thyroid function, dehydration or other constipating medicines, and, for some people, prescribing a medicine that blocks opioid receptors in the gut specifically. Which of these is suitable depends on the individual and on the indication, such as cancer pain, palliative care or chronic non-cancer pain.

In 2020 the MHRA advised that opioids used long-term for non-cancer pain can bring dependence risks and that treatment should have an agreed plan, regular review and slow tapering rather than abrupt stopping. Constipation is one of the reasons for periodic review of whether the benefit still outweighs the burden.

See a GP, or contact the team that manages your pain, if constipation lasts beyond a few days despite laxatives, or comes with bloating, blood in the stool, unexplained weight loss or tiredness. Seek urgent help through 111 or A&E if there is severe or worsening abdominal pain, repeated vomiting, a swollen tummy with no passage of wind or stool, or a high temperature, as these can signal a blockage. Overuse of laxatives can also cause dehydration and low potassium, so persistent muscle weakness or diarrhoea should be reported.