how to deal with constipation caused by taking cocodamol

The short version: opioid-induced constipation is a mechanical problem of slowed gut transit, but the drugs used to treat it come with their own cost-benefit calculation, and most of the evidence comes from industry-funded trials.

The codeine in co-codamol binds to mu-opioid receptors in the gut wall, which slows down how quickly food and stool move through your intestines. That is not a side effect that happens to some people—it is a direct pharmacological consequence. Studies in mice show that morphine delays the time it takes for a bead to move through the gut from about 10 units down to less than 6 [7]. The longer you take it, the more the peripheral receptors in the colon drive the problem, not the central ones [7]. So the mechanism is clear: you are slowing your gut on purpose.

If you want to stay on the drug, the medical system has two classes of solution. The cheap and obvious one is traditional laxatives: stool softeners, fiber, osmotic agents like polyethylene glycol. The 2019 JAMA review recommends those as first-line [5], but the evidence for them in opioid-induced constipation specifically is thin—the same review notes that most of the trials testing laxatives directly against placebo are barely useful. The better-studied class is the peripherally acting mu-opioid receptor antagonists, PAMORAs, which are drugs that block the opioid receptor in the gut without affecting the pain relief in the brain. They work: naldemedine got about 48–52% of people having spontaneous bowel movements compared to 34% on placebo in the COMPOSE-1 and 2 trials [1]. Methylnaltrexone caused 48% of people to have a bowel movement within 4 hours of the first dose, versus 15% on placebo, in a 2008 NEJM trial [4]. Naloxegol doubled the response rate compared to placebo in two large phase 3 trials [2]. But here is the catch: these drugs themselves cause abdominal pain, diarrhea, and nausea at rates that are not trivial. In the COMPOSE-3 long-term trial of naloxegol, diarrhea hit 11% of people versus 5% on placebo, and abdominal pain hit 8% versus 3% [2].

The mouse study on intermittent fasting is interesting—it reduced constipation and even improved pain relief [6]—but it is a mouse study and cannot be a clinical recommendation yet. The one thing that is worth saying: if you are on a short course of cocodamol for acute pain, the constipation is almost always reversible once you stop. If you are on it long-term, the risk of chronic constipation is high, and the drugs to treat it have their own burden of side effects. The safe approach is aggressive use of diet (warm liquids, fiber) and a stool softener like polyethylene glycol before you step up to a prescription PAMORA. The risk of more dangerous outcomes—fecal impaction, bowel obstruction—is low but real in people who take opioids and do not manage constipation at all.

My call: start with diet and a gentle stool softener. If that fails and you need to stay on the drug, PAMORAs are a proven option, but do not assume they are risk-free since those who funded the big trials likely make the products. Confidence: high that the mechanism is real and that PAMORAs improve bowel frequency; low on whether the side effects are worth it for an individual patient given an industry-funded evidence base.