Eli5: How do opiates cause constipation?


My research has turned up reports that it slows bowel activity and causes too much water to be absorbed, but what does that have to do with relieving pain?

In: 6

The slow bowel movement is a side effect. Opioids work on the opioid receptors which are proteins in the central nervous system. They essentially stop your nerves from sending pain signals to the brain.

it has nothing to do with relieving pain.Thats why they’re called side effects, they are an unwanted effect beyond the actual intended effect of the drug

you’ve outlined the specific reasons they do in your own question

Think of your nervous system like your body’s Post Office. It’s responsible for getting messages from one part of your body to the other.

When your brain gets a lot of pain signals from your nervous system, it’s like it’s getting too much junk mail from the post office. Opiates are like asking the post office to stop sending junk mail, except instead of just filtering out certain messages, the post office shuts down a whole distribution center. Unfortunately, we don’t understand our body’s post office well enough to be more specific in our request.

This means you’re not receiving the junk mail as often, but it also slows down other areas of mail delivery. One of the important messages that gets blocked is signals to your bowels. This slows down how food/poo moves through your body, and gives your intestines a much longer time to absorb water out of them.

The body produces its own opioids (“endogenous opioids”) that bind a variety of receptors (eg mu, kappa, delta receptors) to produce a number of effects. One of these effects is analgesia (reduction of pain). When we consume exogenous opioids, they’ll bind to any opioid receptor they get access to. This results not only in analgesia, but also a number of “side effects”.

Constipation is produced because opioids bind opioid receptors in the gut wall to reduce its contractions that are responsible for pushing food forwards (“peristalsis”). The more specific answer is that opioids bind mu receptors in the gut, which leads to decreased release of acetylcholine from the enteric nervous system. Acetylcholine increases gut peristalsis and secretions, so its inhibition leads to constipation.

Constipation can be problematic in patients on long term opioids for pain control (eg cancer patients). Opioids act on the central nervous system (brain & spinal cord) to cause analgesia. Opioids in such cases are often combined with opioid antagonists that cant enter the central nervous system. Thus these opioid antagonists can bind the opioid receptors on the gut, preventing opioids from causing constipation, but are unable to bind the central receptors that opioids act on to reduce pain.