Constipation is most often a sign that something relatively simple needs adjusting, like your fiber intake, hydration, or activity level. But it can also signal underlying medical conditions ranging from thyroid problems and hormonal shifts to neurological disorders and, in rare cases, cancer. About 12% of the global population meets the clinical criteria for chronic constipation, so if you’re dealing with this, you’re far from alone.
Clinically, constipation means fewer than three spontaneous bowel movements per week, or straining, hard stools, and a feeling of incomplete emptying during more than a quarter of your bathroom visits. When these symptoms persist for three months or longer, it’s considered chronic, and that’s when the underlying cause becomes worth investigating.
Low Fiber and Dehydration
The most common reason for constipation is simply not getting enough fiber or water. Current dietary guidelines recommend 14 grams of fiber for every 1,000 calories you eat, which works out to roughly 25 to 30 grams a day for most adults. Most people fall well short of that. Fiber works by adding bulk to stool and helping it move through your large intestine. Without it, stool sits longer, loses water, and becomes harder to pass.
Dehydration compounds the problem. Your colon absorbs water from digested food, and when your body is short on fluids, it pulls more water out of stool than usual. The combination of low fiber and inadequate hydration accounts for a large share of occasional constipation, especially in people whose diets are heavy on processed foods. Physical inactivity plays a role too: regular movement stimulates the natural contractions of your intestines.
Medication Side Effects
Several widely used medications cause constipation as a direct side effect. Opioid painkillers are the most well-known culprit. They bind to receptors in the gut wall that normally help coordinate intestinal movement, and the result is a three-part slowdown: intestinal contractions become weaker, the gut produces less fluid to keep things moving, and the sphincter muscles tighten. This type of constipation, often called opioid-induced constipation, affects the majority of people taking opioids regularly and usually doesn’t improve on its own as long as the medication continues.
Other medications that commonly slow things down include certain antidepressants, blood pressure drugs (especially calcium channel blockers), iron supplements, antacids containing aluminum, and antihistamines. If your constipation started around the same time as a new prescription, that connection is worth raising with your doctor.
Thyroid Problems
Constipation is one of the hallmark symptoms of an underactive thyroid. When your thyroid gland doesn’t produce enough hormones, nearly every system in your body slows down, including your digestive tract. The mechanism involves a buildup of certain substances in the smooth muscle and connective tissue of the gut wall, which impairs the intestine’s ability to contract and push food along. Some people with hypothyroidism also experience delayed stomach emptying, meaning food takes longer to leave the stomach before it even reaches the intestines.
If your constipation comes alongside fatigue, weight gain, dry skin, feeling cold all the time, or hair thinning, those are classic signs of low thyroid function. A simple blood test can confirm or rule it out, and treatment with thyroid hormone replacement typically resolves the digestive symptoms along with the others.
High Calcium Levels
Elevated calcium in the blood, a condition called hypercalcemia, is another metabolic cause of constipation that often goes unrecognized. Calcium plays a central role in how muscles contract, and when levels run too high, it disrupts the coordinated squeezing of the intestinal walls. The result is sluggish digestion, stomach pain, nausea, and constipation. Hypercalcemia is most commonly caused by overactive parathyroid glands or certain cancers, but it can also result from taking too much calcium or vitamin D in supplement form.
Diabetes
Long-standing diabetes, particularly when blood sugar has been poorly controlled over time, can damage the nerves that regulate digestion. This nerve damage (a form of neuropathy) slows the movement of food through the entire digestive tract. Constipation is one of the most frequently reported gut symptoms among people with diabetes, and it tends to develop gradually as nerve function declines over years.
Neurological Conditions
Constipation can be an early warning sign of certain neurological diseases, most notably Parkinson’s disease. The autonomic nervous system, which controls involuntary functions like digestion, is affected early in Parkinson’s. Constipation often appears years before the tremor, stiffness, and movement difficulties that lead to a formal diagnosis. Research suggests it may be one of the earliest detectable symptoms of the disease.
Multiple sclerosis, spinal cord injuries, and stroke can also cause constipation by disrupting the nerve signals that coordinate bowel function. In these cases, constipation is typically one symptom among several neurological changes.
Pelvic Floor Dysfunction
Sometimes constipation isn’t caused by slow movement through the intestines at all. Instead, the muscles of the pelvic floor, which need to relax in a coordinated way to allow a bowel movement, tighten when they should be releasing. This creates a mechanical blockage at the exit. Experts estimate that up to half of people with long-term constipation also have some degree of pelvic floor dysfunction.
Signs that point toward this as the cause include needing to strain excessively, feeling like stool is stuck or blocked, having to press on your abdomen or change positions on the toilet, and frequently feeling like you haven’t fully emptied. Pelvic floor dysfunction is treatable with specialized physical therapy that retrains these muscles, and many people see significant improvement.
Colorectal Cancer
Constipation alone is rarely caused by colorectal cancer, but a sudden change in bowel habits that persists for more than a few days warrants attention, especially when it comes with other warning signs. A tumor growing in the colon or rectum can physically narrow the passage, making stools thin or ribbon-like and increasingly difficult to pass.
Red flags that distinguish this from ordinary constipation include blood in the stool (which can appear bright red or make stools very dark and tarry), unexplained weight loss, increased mucus in stool, and worsening constipation that doesn’t respond to the usual remedies. In advanced stages, stools may become pellet-like or bowel movements may stop altogether. These symptoms don’t automatically mean cancer, but they do call for prompt evaluation.
Irritable Bowel Syndrome
Constipation-predominant IBS is one of the most common functional gut disorders, meaning the digestive system isn’t working properly even though there’s no visible structural problem. People with IBS typically experience constipation alongside bloating, abdominal pain that improves after a bowel movement, and symptoms that fluctuate over time. Stress, certain foods, and hormonal changes (particularly around menstruation) tend to make it worse. IBS is diagnosed after other causes have been ruled out and symptoms have persisted for at least six months.
Pregnancy and Hormonal Shifts
Rising progesterone levels during pregnancy relax smooth muscle throughout the body, including the intestines, which slows transit time. The growing uterus also physically compresses the bowel in later trimesters. Constipation affects a significant percentage of pregnant women and is considered one of the most common gastrointestinal complaints of pregnancy. Hormonal fluctuations around the menstrual cycle can cause similar, milder effects: many women notice their bowel habits shift in the days before their period.
When the Cause Matters Most
Occasional constipation from a low-fiber day or travel-related dehydration resolves on its own and rarely signals anything concerning. The pattern to pay attention to is constipation that’s new, persistent, or worsening, particularly if it arrives alongside other symptoms like unintended weight loss, blood in stool, severe fatigue, or neurological changes. In those cases, constipation may be the most visible piece of a larger picture that’s worth investigating sooner rather than later.