Constipation in adults usually comes down to one or more of a few common triggers: not enough fiber, not enough water, too little physical activity, or a medication side effect. About 10% of adults worldwide meet the clinical criteria for chronic functional constipation, making it one of the most common digestive complaints. While the cause is often straightforward and fixable, constipation can also signal an underlying medical condition, a hormonal shift, or a problem with the muscles that control bowel movements.
How Diet Drives Most Cases
Fiber is the single biggest dietary factor. It works through several mechanisms at once: it holds water inside the stool to keep it soft, it feeds the bacteria in your colon (which adds bulk), it speeds up how quickly waste moves through your intestines, and it stimulates the muscular contractions that push stool along. When your diet is low in fiber, stool becomes small, dry, and slow-moving, which is the textbook setup for constipation.
Current guidelines recommend 14 grams of fiber for every 1,000 calories you eat. For most adults, that works out to roughly 25 to 30 grams per day. Most people fall well short of that number. Good sources include beans, lentils, whole grains, vegetables, fruits, and nuts. If you’re adding fiber to your diet, increase it gradually over a week or two to avoid bloating and gas.
Water matters just as much. Your colon absorbs water from stool as it passes through. If you’re not drinking enough fluid, or if you’re losing extra fluid through sweat or illness, your colon pulls more water out of the stool than it should, leaving it hard and difficult to pass.
Physical Inactivity and Sedentary Habits
Movement helps your gut move. Physical activity stimulates the natural contractions of the intestinal walls that push waste forward. Prolonged sitting or bed rest, whether from a desk job, recovery from surgery, or aging-related immobility, slows those contractions down. Even moderate activity like daily walking can make a noticeable difference in bowel regularity.
Medications That Slow the Gut
A long list of common medications can cause or worsen constipation. The most frequent culprits include:
- Opioid pain medications (codeine, oxycodone, morphine), which directly slow intestinal contractions
- Antacids containing calcium or aluminum
- Antidepressants, particularly older types
- Some blood pressure medications
- Antihistamines used in cold and allergy medicines
- Iron and calcium supplements
If you started a new medication around the time your constipation began, that connection is worth discussing with your prescriber. Opioids are especially notorious for this. Nearly everyone who takes opioids regularly develops some degree of constipation, and unlike most other opioid side effects, the body doesn’t adjust to it over time.
Ignoring the Urge to Go
Habitually holding it in, whether because of a busy schedule, discomfort using public restrooms, or simply not prioritizing it, trains your body in the wrong direction. The longer stool stays in the colon, the more water gets absorbed from it, making it harder and more difficult to pass. Over time, the colon stretches to accommodate the retained stool, and the nerves that signal when it’s time to go become less sensitive. This creates a cycle where you feel the urge less often, stool builds up further, and the problem compounds.
Hormonal Changes
Hormones have a direct effect on how fast your digestive system moves. Pregnancy is the most common example. Rising levels of progesterone relax smooth muscle throughout the body, including the muscles of the intestinal wall, which slows digestion and leads to constipation. Many pregnant people notice this within the first trimester, and it can persist throughout pregnancy, especially as the growing uterus puts physical pressure on the intestines.
The menstrual cycle produces similar, milder effects. Progesterone rises in the second half of the cycle (after ovulation), and many women report constipation during that window, followed by looser stools once their period begins and progesterone drops.
Medical Conditions Behind Chronic Cases
When constipation is persistent and doesn’t respond to diet and lifestyle changes, an underlying condition may be involved. Several systemic diseases affect the nerves, muscles, or hormones that control bowel function:
- Diabetes can damage the nerves that regulate intestinal movement, a complication called autonomic neuropathy
- Hypothyroidism (underactive thyroid) slows metabolism across the board, including digestion
- Parkinson’s disease affects the nerve pathways that coordinate gut motility, and constipation often appears years before the more recognizable motor symptoms
- Multiple sclerosis and other neurological conditions can disrupt the nerve signals between the brain and the digestive tract
- Irritable bowel syndrome (IBS) with constipation is a functional disorder where the gut-brain connection misfires, leading to irregular bowel patterns
Pelvic Floor Dysfunction
Sometimes the problem isn’t what’s happening in the colon. It’s what happens at the exit. Dyssynergic defecation is a condition where the muscles of the pelvic floor fail to coordinate properly during a bowel movement. Normally, when you bear down, the muscles around the rectum relax to let stool pass. In people with this condition, those muscles tighten instead of relaxing, or they can’t generate enough force to push stool out effectively. The result is a feeling of straining, incomplete evacuation, or needing to use manual pressure to pass a bowel movement.
This is more common than most people realize and is frequently misdiagnosed as ordinary constipation. Testing involves a balloon expulsion test, where a small balloon inflated with warm water is placed in the rectum and you’re asked to expel it. If it takes longer than a minute, that suggests pelvic floor dysfunction. Imaging studies can also watch the internal mechanics of defecation in real time. The good news is that biofeedback therapy, which retrains the pelvic floor muscles to coordinate correctly, is effective for most people with this condition.
Symptoms That Need Prompt Attention
Most constipation is uncomfortable but not dangerous. However, certain symptoms alongside constipation are red flags that warrant prompt evaluation. These include blood in the stool, unintended weight loss of 10 pounds or more, iron deficiency anemia, or a new onset of constipation in someone over 50 who previously had normal bowel habits. A family history of colon cancer also raises the level of concern. Any of these may prompt a colonoscopy or other evaluation to rule out a structural problem like a tumor or blockage.
Constipation that comes on suddenly and severely, especially with abdominal pain, vomiting, or a complete inability to pass gas, could indicate a bowel obstruction, which is a medical emergency.