Constipation happens when stool moves too slowly through the digestive tract, giving the colon extra time to absorb water and leaving waste dry, hard, and difficult to pass. Roughly 10 to 15 percent of adults worldwide experience it as a chronic problem, making it one of the most common digestive complaints. The causes range from what you eat and drink to medications, hormones, and everyday habits you might not think twice about.
Your Colon Absorbs Too Much Water
The large intestine’s main job is pulling water out of digested food before it exits the body. When everything works well, just enough water is absorbed to form a soft, easy-to-pass stool. But when waste lingers in the colon longer than it should, the colon keeps extracting water, and stool becomes progressively harder and more compact. Almost every cause of constipation traces back to this basic mechanism: something is slowing transit, and the colon dries things out in the meantime.
Low Fiber Intake
Fiber is the structural part of plant foods that your body can’t digest, and it plays two distinct roles in keeping stool moving. Insoluble fiber, found in whole grains, nuts, and vegetable skins, adds bulk to stool and physically pushes material through the intestines. Soluble fiber, found in oats, beans, and fruits, dissolves in water and forms a gel-like substance that helps stool hold onto moisture so it stays soft.
Most people don’t eat nearly enough of either type. Current dietary guidelines recommend about 14 grams of fiber for every 1,000 calories you eat, which works out to roughly 25 to 35 grams a day for most adults. The average intake in Western diets falls well short of that. Without enough fiber, stool loses both its bulk and its ability to retain water, and the result is small, hard pieces that are slow to move and painful to pass.
Not Drinking Enough Water
Water softens food as it moves through the digestive tract and keeps stool pliable enough to pass without straining. When you’re even mildly dehydrated, your body compensates by pulling more fluid from the colon, which leaves stool drier and harder. This is why constipation often gets worse in hot weather, after intense exercise, or when you’re sick with a fever. Drinking more water won’t cure constipation caused by other factors, but chronic under-hydration reliably makes it worse.
Medications That Slow the Gut
A surprisingly long list of common medications can cause constipation, and many people don’t connect the two. The worst offenders are prescription painkillers in the opioid family, which slow the nerves in the gut and can reduce bowel movements dramatically. But plenty of everyday drugs do the same thing through different pathways:
- Blood pressure medications. Calcium channel blockers relax the muscles in blood vessel walls, but they also relax intestinal muscles. Beta-blockers can slow the digestive system as well.
- Antidepressants. Certain SSRIs and older tricyclic antidepressants interfere with the chemical signals that keep the gut contracting rhythmically.
- Allergy medications. Antihistamines like diphenhydramine (the active ingredient in Benadryl) block a chemical called acetylcholine that helps intestinal muscles move.
- Iron supplements. These are well known for causing digestive slowdowns, particularly at higher doses.
- Antacids. Heartburn remedies containing calcium or aluminum can harden stool.
- Bladder medications. Drugs prescribed for urinary incontinence affect muscle movement throughout the body, including the intestines.
Even over-the-counter pain relievers like ibuprofen and naproxen occasionally contribute. If constipation started around the same time as a new medication, that connection is worth exploring.
Hormonal Changes During Pregnancy
Constipation is extremely common during pregnancy, and hormones are the primary reason. Progesterone, which rises sharply to support the pregnancy, directly inhibits the contractions of intestinal smooth muscle. It does this by shifting the balance of chemical signals in the gut wall: it dials down the signals that trigger contraction and amplifies the ones that promote relaxation. The result is a slower, more sluggish digestive tract that gives the colon extra time to absorb water from stool.
This isn’t limited to pregnancy. Progesterone fluctuations during the menstrual cycle explain why many women notice constipation in the days before their period, when progesterone peaks. The physical pressure of a growing uterus on the intestines and the iron supplements commonly prescribed during pregnancy compound the problem further.
Ignoring the Urge to Go
When you feel the urge to have a bowel movement, it means stool has reached the rectum and stretch receptors are signaling your brain that it’s time. If you repeatedly ignore that signal, whether because of a busy schedule, discomfort using public restrooms, or just inconvenient timing, the stool sits in the rectum and continues to lose water. Over time, habitual suppression can dull those stretch receptors, meaning you need a larger, harder stool to trigger the urge at all.
Chronic straining to pass hard stool can also deteriorate the muscles and nerves involved in defecation, according to Cleveland Clinic, creating a cycle where the problem reinforces itself. The pelvic floor muscles that need to relax during a bowel movement can become uncoordinated, making it harder to evacuate even when stool is present.
Physical Inactivity
Movement stimulates the rhythmic contractions of the intestines, called peristalsis, that push waste forward. People who are sedentary, whether from a desk job, bed rest after surgery, or limited mobility with age, tend to have slower transit times. This is one reason constipation rates climb in older adults: reduced physical activity, combined with age-related changes in nerve signaling and a higher likelihood of taking constipating medications, creates a perfect storm. Even moderate daily activity like walking can meaningfully improve bowel regularity.
Stress and Disrupted Routines
The gut has its own extensive nervous system, sometimes called the “second brain,” and it responds to stress just as the rest of your body does. Psychological stress can alter gut motility in either direction, speeding it up (causing diarrhea) or slowing it down (causing constipation). Travel is a classic trigger because it combines multiple factors at once: disrupted sleep, changes in diet, dehydration from flying, unfamiliar bathrooms, and shifted time zones that throw off your body’s internal schedule for bowel movements.
Shift work, jet lag, and irregular meal timing can all interfere with the gastrocolic reflex, which is the natural increase in intestinal activity that happens after eating. When your eating schedule is erratic, that reflex becomes less predictable, and bowel movements become less regular.
Underlying Medical Conditions
Sometimes constipation is a symptom of something else happening in the body. An underactive thyroid slows metabolism broadly, including gut motility. Diabetes can damage the nerves that control intestinal contractions over time. Neurological conditions like Parkinson’s disease and multiple sclerosis can impair the nerve pathways between the brain and the gut. Pelvic floor dysfunction, where the muscles that coordinate defecation don’t relax properly, is another common and underdiagnosed cause, particularly in women after childbirth.
Persistent constipation that doesn’t respond to diet changes, more water, and increased activity, or that comes with new symptoms like blood in the stool or unexplained weight loss, points toward something beyond lifestyle factors.