Why Do Opioids Cause Constipation?

It's just a side effect right? Except it's one that's affecting a lot of people. Miroslav Kliewksi/CSA Images/Getty/iStock

It's easy to snicker at the ads.

You know, the ones with dripping paint symbolizing slow bowels, or a man forlornly watching a dog doing its business on the street. Nearly 112 million people, in fact, probably snickered at the same time when one of the ads ran during Super Bowl 50.

But it wasn't just a weird ad buy from two pharmaceutical companies (Daiichi-Sankyo and AstraZeneca) with money to burn. With opioid usage and abuse at an alarming high, the side effects of opioid use have become a growing concern. Treating those side effects has become a thriving industry. Prince's recent death from an overdose of the opioid painkiller fentanyl has also shined a high-profile light on the myriad problems that these pain medications pose.

But why exactly do opioids make your insides skid to a slow crawl? The answer lies in how opioids work to combat discomfort. When you're in pain and take an opioid drug, the opioids are going to bind to receptors in your brain and spinal cord. They create an analgesic effect, reducing your perception of pain — exactly what someone suffering is looking for in a medication.

However, these opioid receptors aren't just located in the central nervous system. They're scattered throughout the body, including the gastrointestinal tract. And when you pop a pill, these opioid receptors in your gut (especially the submucosa and ileal mucosa in your stomach and intestines) are going to be affected.

The neurons react in several ways, but it all adds up to creating a sluggish GI tract that doesn't have the robust propulsion and mobility that a regularly functioning one does. Opioids also cause the body to absorb more fluids, which leaves you with a dry stool.

So while opioids are dutifully binding to central and peripheral nervous system receptors to alleviate pain, they're also making neurons all over the body behave differently. It's this slowdown that causes opioid constipation — and the glut of ads you've seen recently for medications to combat it.