Does Acetaminophen Cause Constipation?

Acetaminophen (known as paracetamol outside North America) is not a meaningful cause of constipation when taken alone at standard over-the-counter doses. The confusion almost always traces back to combination products that pair acetaminophen with an opioid like codeine or hydrocodone, and those opioid ingredients are well-established constipation culprits. The distinction matters because millions of people reach for plain acetaminophen every day, and misattributing a side effect to the wrong ingredient can lead to unnecessary avoidance of a generally well-tolerated pain reliever or, worse, a failure to address the actual source of the problem.

What Long-Term Trials Actually Show

The best place to look for whether a drug causes constipation is in controlled trials where patients take it for months and researchers systematically track side effects. In a large, year-long trial comparing acetaminophen to naproxen in adults with osteoarthritis, constipation was reported by about 3% of people taking acetaminophen. By contrast, roughly 10% of those on naproxen reported constipation, a statistically significant difference in naproxen’s disfavor.¹ A 3% rate over six to twelve months of daily use is low, and some of that likely reflects baseline constipation that would occur with or without the drug. In clinical trial methodology, background noise like this is expected; it does not by itself prove the drug is responsible.

A review article on acetaminophen use in pregnancy did list constipation among documented side effects at low doses, alongside nausea and vomiting.² So the possibility isn’t zero. But the rate is low enough, and the evidence thin enough, that no major clinical guideline singles out constipation as a notable risk of taking acetaminophen by itself. When regulatory agencies and medical bodies talk about acetaminophen’s side-effect profile, liver toxicity at high doses dominates the conversation. Constipation is, at most, a footnote.

The Real Culprit in Combination Products

Here is where the misattribution problem gets serious. Many prescription pain medications pair acetaminophen with an opioid, and these combination products carry constipation rates that dwarf anything seen with acetaminophen alone. In a 23-day trial of cancer patients taking either codeine/acetaminophen or hydrocodone/acetaminophen, constipation hit 36% in the codeine group and 29% in the hydrocodone group.³ That is roughly ten times the rate seen with acetaminophen alone in the osteoarthritis trial. A similar pattern showed up in elderly patients with osteoarthritis flare-ups who took tramadol combined with acetaminophen: constipation occurred in about 4% of them, with the other common side effects (nausea, vomiting, dizziness) pointing squarely at tramadol’s opioid activity.⁴

Opioids cause constipation through a well-understood mechanism: they bind to receptors in the gut wall that slow down the muscle contractions pushing food through your intestines. This effect is so reliable that opioid-induced constipation has its own medical abbreviation (OIC) and its own class of prescription drugs designed to counteract it. When someone takes a pill labeled “acetaminophen with codeine” and develops constipation, the label says acetaminophen, the bottle says acetaminophen, and it’s natural to blame the drug you recognize. But the codeine is doing the heavy lifting on the constipation front.

Acetaminophen’s Effect on Gut Motility

If acetaminophen were a strong constipation agent, you’d expect it to slow down contractions in the colon the way opioids do. Researchers tested this directly by exposing human colon tissue to paracetamol in a laboratory setting. At concentrations that match what circulates in the body after a normal dose, paracetamol did not change spontaneous colonic contractions or responses to stimulation at all. Only at concentrations hundreds of times higher than therapeutic levels did it start to dampen those contractions.⁵ The takeaway is that your colon effectively ignores acetaminophen at the doses you’d take for a headache or a sore knee. The drug simply doesn’t interact with gut motility in a clinically relevant way at standard doses.

This fits with what we know about how acetaminophen works. Its primary action is in the central nervous system, where it modulates pain signaling and fever regulation. Unlike opioids, which lock onto receptors throughout the gut, or NSAIDs, which inhibit prostaglandins involved in mucosal protection and gut function, acetaminophen’s footprint in the gastrointestinal tract is relatively light. It gets absorbed in the small intestine and metabolized in the liver, and along the way, it doesn’t dramatically alter how the gut moves things along.

How Acetaminophen Compares to Other Over-the-Counter Pain Relievers

When researchers line up common OTC analgesics and compare their gastrointestinal side effects, acetaminophen consistently ranks among the gentlest options. One study found that overall GI side-effect rates at OTC doses were about 7.6% for acetaminophen, 7.2% for ibuprofen, and 37% for diclofenac. GI effects accounted for three-quarters of all adverse events in that study.⁶ A separate trial comparing ibuprofen, paracetamol, and aspirin at standard OTC doses for up to a week found GI adverse event rates of about 11.5% for ibuprofen, 13% for paracetamol, and 18.5% for aspirin. The difference between ibuprofen and paracetamol was not statistically significant, while aspirin was clearly worse.⁷

Note that these GI side effects are mostly things like stomach discomfort, nausea, and diarrhea rather than constipation specifically. NSAIDs such as ibuprofen and aspirin are known to cause stomach pain and ulcers because they reduce the protective prostaglandins lining the stomach. Acetaminophen largely avoids that particular problem, which is why doctors often recommend it for people with sensitive stomachs or a history of ulcers. The point here is that if you’re choosing a pain reliever and constipation is a concern, acetaminophen by itself is about as benign as your options get.

IV Acetaminophen in Hospital Settings

Intravenous acetaminophen is commonly used in hospitals after surgery, partly because it can be given to patients who can’t swallow pills and partly because the hope is that it will reduce the amount of opioids a patient needs. Less opioid use would, in theory, mean less opioid-induced constipation, nausea, and sedation. In practice, the opioid-sparing effect has been inconsistent. A systematic review of surgical patients found that IV acetaminophen produced a meaningful reduction in opioid use in only about a third of the groups studied, and overall, it was not more effective at cutting opioid consumption than other non-opioid pain treatments.⁸

One trial looking at patients undergoing major abdominal surgery found that scheduled IV acetaminophen did not reduce total opioid consumption compared with placebo when both groups had access to the same patient-controlled opioid pumps and other pain medications.⁹ Why does this matter for constipation? Because when IV acetaminophen doesn’t meaningfully cut opioid use, the constipation problem that patients experience after surgery remains driven by the opioids they’re still receiving. The acetaminophen itself isn’t contributing to the constipation, but it may not be doing as much as hoped to prevent it indirectly, either.

An interesting wrinkle: in a pediatric study comparing IV acetaminophen with oral acetaminophen for fever, about 4% of children in the IV group developed constipation, while none in the oral group did.¹⁰ This was a small finding in one study and not necessarily generalizable, but it raises the question of whether the route of administration or the hospital setting itself (reduced mobility, altered diet, stress) plays a role. Hospitalized children who are receiving IV medications are, by definition, sicker or more limited in activity than those able to swallow pills, and immobility alone is a well-known constipation trigger.

Acetaminophen and the Gut Microbiome

A newer line of research looks at how acetaminophen interacts with the community of microbes living in the gut. In mice, altering the gut microbiome with antibiotics changed how acetaminophen was absorbed and processed in the body. The study found that shifts in microbial composition were associated with changes in acetaminophen biodisposition, meaning the drug was handled differently depending on which bacteria were present.¹¹ This is a two-way street: the microbiome influences how your body processes acetaminophen, and acetaminophen exposure may, in turn, affect the microbiome.

In humans, a birth cohort study found that children whose mothers used acetaminophen during pregnancy had lower gut microbial diversity compared with unexposed children. Specifically, prenatal acetaminophen exposure was linked to lower levels of Firmicutes (one of the dominant bacterial groups in a healthy gut) and higher levels of Actinobacteria.¹² The practical significance of this for everyday acetaminophen use and bowel habits remains unclear. A less diverse microbiome has been loosely linked to various digestive issues in broader research, but no one has drawn a direct line from “took acetaminophen, microbiome shifted, got constipated.” This is early-stage science that raises questions rather than answering them.

When Acetaminophen Does Hurt the Gut

There is one scenario where acetaminophen causes dramatic gastrointestinal symptoms, and it’s one you want to avoid entirely: overdose. In a study of nearly 300 acetaminophen poisoning cases, more than three-quarters of patients had GI symptoms on arrival at the hospital. The presence of those symptoms (which include nausea, vomiting, and abdominal pain rather than constipation specifically) predicted worse outcomes including liver and kidney damage.¹³ The risk was especially elevated when the ingested dose reached 10 grams or more, which is more than double the maximum recommended daily dose for adults. At those levels, acetaminophen overwhelms the liver’s capacity to metabolize it safely, and the resulting toxic metabolites wreak havoc.

The key distinction is that overdose-level GI distress is an entirely different phenomenon from the mild, background-rate constipation occasionally reported in clinical trials. One is a medical emergency; the other is a minor nuisance that may not even be caused by the drug. If you’re taking acetaminophen within the labeled dose (no more than 3,000 to 4,000 milligrams per day for most adults, and less if you drink alcohol regularly or have liver concerns), acute GI toxicity is not on the table.

Practical Advice for People Experiencing Constipation While Taking Acetaminophen

If you’ve noticed constipation that seems to coincide with taking acetaminophen, the first thing to check is whether your product contains another active ingredient. Flip the box over and look at the active ingredients list. Brand names can be misleading: products marketed under acetaminophen-associated brands (like certain Tylenol formulations) sometimes contain codeine, tramadol, oxycodone, or other opioids. Prescription combination products are especially common after dental procedures, surgeries, and for chronic pain management. If you see any opioid ingredient listed, that is almost certainly the cause of your constipation.

If you’re genuinely taking plain acetaminophen and experiencing constipation, consider other likely factors. Reduced physical activity (common when you’re in enough pain to take a painkiller), changes in diet or fluid intake, stress, and other medications can all slow the gut. Iron supplements, certain blood pressure medications, and antacids containing aluminum are well-known constipation triggers that people sometimes take alongside acetaminophen without connecting the dots.

For people who need regular pain relief and are prone to constipation, acetaminophen alone remains one of the safer choices among common analgesics. It lacks the direct gut-slowing effect of opioids and the stomach-irritating properties of NSAIDs. If constipation persists regardless of what you’re taking, the usual lifestyle measures apply: adequate water intake, fiber-rich foods, regular movement, and, if needed, a conversation with your doctor about whether an underlying condition or another medication is the real source of the problem.

Why This Misconception Persists

Part of the staying power of the “acetaminophen causes constipation” belief comes from the sheer ubiquity of combination products. In the United States alone, acetaminophen appears in hundreds of products, from cold medicines to prescription opioid formulations. When people develop constipation and check which drug they’ve been taking, acetaminophen’s name is right there on the label, often in larger print than the opioid it’s paired with. Over time, this creates a strong informal association in people’s minds.

Another factor is that the conditions leading people to take acetaminophen frequently involve immobility. A person with a bad back, recovering from surgery, or dealing with arthritis pain may be spending more time sitting or lying down. Reduced physical activity is one of the most reliable predictors of constipation, and it has nothing to do with the pill they swallowed. The pain drove the pill-taking, and the pain also drove the inactivity that caused the constipation. Untangling that kind of correlation from causation is exactly the sort of thing clinical trials are designed to do, and the trial data, as reviewed above, does not support acetaminophen as a significant constipation risk at normal doses.

There’s also a general tendency to assume that any medication taken by mouth must affect the stomach and intestines in noticeable ways. For some drug classes, that’s absolutely true. But acetaminophen is absorbed quickly from the upper GI tract and processed in the liver, spending relatively little time interacting with the lower bowel where constipation originates. The ex vivo colonic tissue research bears this out: at the concentrations your colon actually sees after a standard dose, the drug is essentially inert with respect to gut motility.⁵