Most prescription muscle relaxers will not help constipation and are more likely to make it worse. The muscle relaxers commonly prescribed for back pain and muscle spasms, like cyclobenzaprine, tizanidine, and methocarbamol, target skeletal muscles rather than the smooth muscle that lines your digestive tract. Since your gut moves waste through smooth muscle contractions, these drugs don’t improve bowel motility. In some cases, they actively slow it down.
There is one notable exception: when constipation is caused by pelvic floor muscles that are too tight, certain muscle relaxants can help. But for garden-variety constipation, reaching for a muscle relaxer is the wrong approach.
Why Skeletal Muscle Relaxers Don’t Help Your Gut
Your body has two fundamentally different types of muscle. Skeletal muscle is what you flex at the gym. Smooth muscle lines your intestines, stomach, and blood vessels, and it operates automatically. Prescription muscle relaxers like cyclobenzaprine, methocarbamol, and tizanidine are designed to act on skeletal muscle, your nervous system, or both. They have very little effect on the smooth muscle that pushes food and waste through your digestive tract.
Research on dantrolene, a skeletal muscle relaxer, illustrates this clearly. In laboratory testing, intestinal smooth muscle required roughly 14 times the drug concentration to show any effect compared to skeletal muscle, and those effects were described as “highly variable and of short duration.” Your gut simply isn’t the target these drugs are built for.
Baclofen, another common muscle relaxer used for spasticity, also shows minimal impact on gut transit. In animal studies, baclofen at standard doses produced no significant change in either gastric or colonic transit times, meaning waste moved through the digestive system at the same pace whether the drug was present or not.
Muscle Relaxers Can Actually Cause Constipation
Not only do these medications fail to help constipation, several of them list it as a known side effect. Cyclobenzaprine causes constipation in 1 to 3 percent of patients taking it. Tizanidine lists constipation as a less common but recognized side effect. The reason is that many of these drugs have mild anticholinergic properties, meaning they partially block a chemical messenger (acetylcholine) that helps stimulate gut contractions. The result is a slower, more sluggish digestive tract.
In rare and extreme cases, the consequences can be more serious. A case report documented a patient who developed paralytic ileus, a complete shutdown of intestinal movement, after receiving baclofen through an implanted pump. The ileus resolved only after the dose was significantly reduced. While this scenario involved an unusual delivery method and a patient with a brainstem injury, it underscores that muscle relaxers can impair gut function rather than enhance it.
GI Antispasmodics Are Different, but Still Not for Constipation
There is a separate category of drugs sometimes called “smooth muscle relaxants” or antispasmodics that do target the gut directly. These include medications like dicyclomine and hyoscyamine. They work by blocking calcium from entering intestinal smooth muscle cells or by inhibiting the nerve signals that trigger gut contractions. They’re used primarily to reduce painful cramping and spasms in conditions like irritable bowel syndrome (IBS).
Here’s the catch: because these drugs reduce intestinal contractions, they tend to slow transit rather than speed it up. Survey data shows that about 30 percent of patients with the diarrhea-predominant form of IBS have used antispasmodics, which makes sense since slowing things down is exactly what those patients need. If you have constipation, an antispasmodic would likely make your situation worse. The 2021 guidelines from the American College of Gastroenterology actually stopped recommending currently available smooth muscle antispasmodics in the United States for IBS treatment altogether, citing limited evidence of benefit.
The Exception: Pelvic Floor Muscle Tightness
There is one specific type of constipation where muscle relaxants can genuinely help. In pelvic floor dysfunction, the muscles at the base of your pelvis are chronically too tight or fail to coordinate properly when you try to have a bowel movement. Instead of relaxing to let stool pass, they clench. This creates a physical blockade that no amount of fiber or water will fix.
For this condition, treatment algorithms recommend pelvic floor physical therapy as the first step. If physical therapy alone doesn’t resolve the problem, second-line options include vaginal muscle relaxant suppositories, trigger point injections, and cognitive behavioral therapy, often used alongside continued physical therapy. These muscle relaxants are applied locally, right where the problem is, rather than taken as a pill that circulates through your whole body. If your constipation involves a feeling of blockage, excessive straining, or a sense that stool is “stuck” despite being soft, pelvic floor dysfunction could be the underlying cause.
Magnesium: The Overlap Between Muscle Relaxation and Bowel Relief
Magnesium is one substance that genuinely bridges the gap between muscle relaxation and constipation relief, though not for the reason most people assume. Magnesium supplements are widely used to ease muscle cramps and tension because magnesium plays a role in how muscles contract and relax. But its effect on constipation works through an entirely different mechanism.
Magnesium-based laxatives (magnesium oxide, magnesium citrate, magnesium hydroxide, also known as milk of magnesia) relieve constipation osmotically. Magnesium ions are poorly absorbed in the gut, so they draw water into the intestinal lumen. This softens stool and increases its volume, which then stimulates the intestinal wall to push things along. The key effect is softening hard stools rather than directly increasing muscle contractions. If your constipation involves hard, dry stools that are difficult to pass, magnesium-based products can be effective.
So while magnesium can help both muscle tension and constipation, it does so through two separate pathways. Taking it for one problem may coincidentally help the other, but the muscle-relaxing property isn’t what’s moving your bowels.
What Actually Works for Constipation
If you’re dealing with constipation while taking a muscle relaxer, the muscle relaxer itself could be contributing. Increasing your water intake, adding fiber gradually, and staying as physically active as your condition allows are practical first steps. Over-the-counter osmotic laxatives, including magnesium-based options, can provide reliable short-term relief. Stimulant laxatives work differently by directly triggering intestinal contractions, but they’re best used occasionally rather than daily.
If your constipation started or worsened after beginning a muscle relaxer, that timing is worth mentioning to your prescriber. Switching to a different medication or adjusting the dose may resolve the issue. And if your constipation involves chronic straining, a sensation of incomplete emptying, or the need to press on your perineum to pass stool, ask specifically about pelvic floor evaluation, since that’s the one scenario where targeted muscle relaxation genuinely treats the root cause.