Does Methocarbamol Cause Constipation?

Constipation is not among the commonly reported side effects of methocarbamol. The drug’s recognized adverse effects center on drowsiness, dizziness, stomach discomfort, and blurred vision, with no specific mention of constipation in standard pharmacology references.¹ That said, the question is not unreasonable: methocarbamol belongs to a drug class where constipation does sometimes show up, and the circumstances in which people take it often involve other constipation-promoting factors worth understanding.

What Methocarbamol Is Designed to Do

Methocarbamol is a centrally acting skeletal muscle relaxant. Rather than working directly on the muscle itself, it acts within the spinal cord and brainstem to dampen the overactive nerve signals that cause muscle spasms. Early animal research showed that methocarbamol, at doses sufficient to abolish certain complex spinal reflexes, left simpler reflexes like the knee-jerk intact, suggesting it targets the multi-step neural pathways involved in spasm without broadly shutting down all nerve transmission.² This selective action on the central nervous system is why drowsiness and dizziness top the list of side effects. The drug is calming the nervous system to reduce spasm, and that sedation spills over into how you feel generally.

Because methocarbamol’s primary activity is in the central nervous system rather than the gut, it does not directly slow intestinal motility the way opioid painkillers do. Opioids bind to receptors in the gut wall and physically slow the muscular contractions that push food along. Methocarbamol does not have that mechanism. Its gastrointestinal effects, when they occur, tend to manifest as nausea or general stomach discomfort rather than slowed transit and hard stools.¹

The Anticholinergic Factor

If methocarbamol doesn’t directly target the gut, why do some people still experience constipation while taking it? The most likely pharmacological explanation involves something called anticholinergic activity. Acetylcholine is a chemical messenger your body uses for many things, including stimulating the muscles that line your digestive tract. When a drug blocks or reduces acetylcholine’s effects, digestion can slow down, leading to constipation, dry mouth, urinary retention, and blurred vision.

Methocarbamol does have some anticholinergic properties, though they are relatively mild compared to other drugs in its class. A study of 260 intensive care unit patients compared anticholinergic side effects between methocarbamol and cyclobenzaprine, another popular muscle relaxant. Among patients receiving methocarbamol, about 11% experienced potential anticholinergic adverse effects. Among those on cyclobenzaprine, that figure was closer to 14%.³ So methocarbamol is not zero-risk on this front, but it sits at the lower end of the spectrum for skeletal muscle relaxants.

It is worth keeping in mind that the anticholinergic category lumps together several different symptoms. Dry mouth, blurred vision, and urinary issues all count. Constipation is one possible manifestation, but not the only or even the most common one for methocarbamol. A person who notices dry mouth on the drug is experiencing anticholinergic effects, but that does not necessarily mean constipation will follow.

How Methocarbamol Compares to Other Muscle Relaxants

Not all muscle relaxants carry the same constipation risk. Cyclobenzaprine, which is structurally related to tricyclic antidepressants, has a notably stronger anticholinergic profile. Its chemical structure gives it a more pronounced tendency to block acetylcholine, which is why dry mouth and constipation are more frequently reported with it than with methocarbamol. The ICU study mentioned above bears this out: cyclobenzaprine produced anticholinergic side effects in a larger proportion of patients than methocarbamol did.³

Tizanidine, another frequently prescribed muscle relaxant, works through a different mechanism (alpha-2 receptors) and has its own set of gastrointestinal effects, including constipation reported in clinical trials. Baclofen, which acts on GABA receptors, also lists constipation among its recognized side effects. Compared to these alternatives, methocarbamol sits in a relatively favorable position when it comes to gut-related complaints. If constipation is a particular concern for you, this difference could matter when discussing options with your prescriber.

What the Large-Scale Adverse Event Data Shows

One useful way to assess whether a drug commonly causes constipation is to look at pharmacovigilance databases, which collect adverse event reports from across the healthcare system. A 2024 study mined the FDA’s Adverse Event Reporting System (FAERS) to identify the 30 drugs most commonly associated with drug-induced constipation and to calculate their risk signals.⁴ Methocarbamol did not appear among those 30 drugs. The list was dominated by opioids, certain antidepressants, antipsychotics, and other medications with well-established effects on gut motility. The absence of methocarbamol from this list does not prove it can never cause constipation in anyone, but it does suggest that constipation is not a prominent signal for this drug at the population level.

Pharmacovigilance data has limitations, of course. Adverse events are voluntarily reported, so mild or expected symptoms may be underreported. A person who gets constipated while taking methocarbamol alongside an opioid might attribute the symptom to the opioid and never file a report mentioning the muscle relaxant. Still, the FAERS database includes millions of reports, and the consistent absence of methocarbamol from constipation-linked drug lists is meaningful.

Why You Might Still Get Constipated While Taking It

Even if methocarbamol itself is a minor player in constipation risk, the circumstances surrounding its use often stack the deck. People typically take methocarbamol for acute musculoskeletal pain, and that context brings along several genuinely constipating factors.

Opioid co-prescription: Methocarbamol is frequently prescribed alongside opioid painkillers for injuries, post-surgical recovery, or severe back pain. Opioids are among the most reliably constipating drugs in existence. If you started methocarbamol and an opioid around the same time and then developed constipation, the opioid is almost certainly the primary culprit.
Reduced physical activity: When you are laid up with a muscle injury or recovering from surgery, you move less. Reduced movement slows gut transit. The pain itself discourages the kind of walking and light activity that normally keeps digestion on track.
Dietary changes: Pain and sedation often reduce appetite, and people in acute pain tend to reach for convenient, low-fiber foods. Less fiber and less water intake slow digestion independently of any drug effect.
Other medications: Beyond opioids, you might be taking anti-inflammatory drugs, anesthetics (if post-surgical), or other medications that affect gut function. The combination of multiple drugs is often more constipating than any single one.

Sorting out which factor is responsible matters because the solution differs. If the constipation is really from an opioid, a stool softener or osmotic laxative targeted at opioid-induced constipation makes sense. If it is from inactivity and low fiber, gradually increasing movement and dietary fiber addresses the root cause. Simply blaming methocarbamol and discontinuing it could leave you without pain relief while the actual constipating factor persists.

Older Adults and Accumulated Anticholinergic Burden

One group that should pay closer attention to even mild anticholinergic effects is older adults. As people age, their bodies become more sensitive to anticholinergic drugs, and many older adults take several medications that each contribute a small anticholinergic load. The combined effect, sometimes called anticholinergic burden, can add up to clinically meaningful symptoms including constipation, confusion, dry mouth, and urinary difficulties. Skeletal muscle relaxants, including methocarbamol, are among the medications flagged for careful use in older populations.⁵

For a younger adult taking only methocarbamol, the drug’s weak anticholinergic properties are unlikely to cause noticeable constipation on their own. For a 75-year-old already on an antihistamine, a bladder medication, and perhaps an antidepressant with anticholinergic properties, adding methocarbamol could be the straw that tips the balance. In those situations, the constipation is not really “caused by methocarbamol” in isolation. It is the aggregate effect of multiple medications acting on the same system. A pharmacist or physician can review the full medication list and assess total anticholinergic load, which is a more useful exercise than blaming any single drug.

Practical Steps If You Notice Constipation

If you develop constipation after starting methocarbamol, here is a reasonable approach before assuming the muscle relaxant is to blame:

Review all your medications: Check whether you are also taking an opioid, an antihistamine, or any other drug known to slow gut motility. Any of these is a more likely cause than methocarbamol.
Assess your activity level: Have you been moving less because of pain or injury? Even short, gentle walks can help restore normal bowel function.
Check hydration and fiber: Methocarbamol can cause some drowsiness, which might reduce how much you drink throughout the day. Staying hydrated and eating adequate fiber are the simplest first interventions.
Talk to your prescriber: If constipation persists and is bothersome, your doctor can evaluate whether switching to a different muscle relaxant, adjusting the dose, or adding a targeted laxative makes sense. Do not stop the medication on your own without discussing it, especially if it is effectively managing your pain.

Over-the-counter options like osmotic laxatives or stool softeners are generally safe for short-term use alongside methocarbamol, but confirming with a pharmacist is reasonable if you are on multiple medications.

The Urine Color Surprise

While constipation gets the worried internet searches, methocarbamol has a different and entirely harmless side effect that catches many people off guard: it can turn your urine black, dark brown, blue, or green.¹ This discoloration is caused by a metabolite of the drug and is completely benign. It does not indicate liver damage, kidney problems, or any kind of internal bleeding. The effect resolves as soon as you stop taking the medication. Plenty of people discover this in the middle of the night and assume something is seriously wrong, so knowing about it in advance saves a lot of unnecessary anxiety and after-hours calls to poison control.

The discoloration can also interfere with certain urine-based laboratory tests, producing false readings for some substances. If you are scheduled for a urinalysis or drug screen, mention that you are taking methocarbamol so the lab can account for it. This is a practical consideration that often goes unmentioned during prescribing but can prevent confusion down the line.

Sedation as the More Relevant Concern

For most people taking methocarbamol, the side effect worth genuinely preparing for is sedation, not constipation. Drowsiness and dizziness are the most commonly reported effects and the ones most likely to affect your daily life. The drug works by calming the central nervous system, and that calming effect is not limited to the spinal cord pathways controlling your tight muscles. It extends to your brain’s alertness systems as well.

Combining methocarbamol with alcohol, benzodiazepines, or other central nervous system depressants amplifies this sedation substantially and can become dangerous. Falls, impaired driving, and slowed reaction times are real risks, particularly in the first few days of treatment while your body is adjusting to the drug. If you are going to worry about a side effect of methocarbamol, drowsiness deserves far more of your attention than constipation does. Planning around the sedation, avoiding driving until you know how the drug affects you, and being cautious on stairs or uneven surfaces are the practical precautions that matter most during a course of treatment.

1
PubMed Central. Use of Muscle Relaxants for Acute Postoperative Pain: A Practical Review
2
PubMed Central. Mephenesin, methocarbamol, chlordiazepoxide and diazepam: actions on spinal reflexes and ventral root potentials
3
PubMed Central. Anticholinergic Adverse Effects Associated With Skeletal Muscle Relaxants in the Intensive Care Unit – Section: Results
4
Frontiers in Pharmacology. Exploring the top 30 drugs associated with drug-induced constipation based on the FDA adverse event reporting system – Section: 3 Results
5
ScienceDirect (Elsevier). Potentially inappropriate medications in geriatric population: a clinical update for oral medicine and orofacial pain practitioners