The condition and the treatment(s)
Although constipation has many causes, it can sometimes be a side effect of taking opiate painkillers (opioids), such as codeine and morphine. As with any kind of constipation, the symptoms of opioid‑induced constipation can vary: you might not pass stools (faeces) as often as you normally do, you might have to strain more than usual or you might be unable to completely empty your bowels.
Naloxegol (Moventig) is a drug that counteracts how the opioids affect your gastrointestinal tract, without making them any less effective at reducing pain.
NHS Choices (www.nhs.uk) may be a good place to find out more.