Does Parkinson’s Disease Cause Diarrhea?

Constipation, not diarrhea, is the signature gut symptom of Parkinson’s disease. In studies of people with early Parkinson’s, fewer than 4% reported loose stools or diarrhea, while nearly half reported constipation. But that statistic hides a more complicated reality. Diarrhea does show up in Parkinson’s patients, sometimes because of the medications used to treat the disease and sometimes because of gut changes the disease itself sets in motion.

Why Parkinson’s Affects the Gut at All

Parkinson’s is usually thought of as a brain disease, and the tremor, stiffness, and slow movement that define it are indeed caused by the loss of dopamine-producing neurons in the brain. But the disease also damages the nervous system that runs independently through the gut wall. Post-mortem studies have found clumps of the protein alpha-synuclein, the same protein that builds up in the brains of Parkinson’s patients, inside neurons of the enteric nervous system as well.¹ Animal research has confirmed that inducing alpha-synuclein buildup in the enteric nervous system is, on its own, enough to cause gut dysfunction.²

This matters because the enteric nervous system coordinates everything from swallowing to the rhythmic contractions that move food through the intestines. When its neurons accumulate abnormal proteins and start malfunctioning, the whole digestive pipeline can slow down. That is why digestive complaints in Parkinson’s are not just an inconvenience but a core feature of the disease: they reflect the same protein pathology that destroys movement-related brain circuits, just in a different location.

Constipation Is the Dominant Problem

When researchers catalog gut symptoms in Parkinson’s patients, the results are lopsided. A study of people with early-stage disease found constipation in about 46%, excessive straining during bowel movements in a third, and trouble relaxing the anal sphincter in roughly 11%. In contrast, loose stools or diarrhea appeared in only about 4%.³ That pattern holds broadly across the literature. Reviews of Parkinson’s-related gut problems consistently list dysphagia, nausea, bloating, and constipation as the most common complaints.⁴

The reason is straightforward. Parkinson’s slows gut motility. The same nerve damage that causes stiffness and slowness in skeletal muscles has an analogous effect on the smooth muscle of the intestines. Food takes longer to travel through the system, water gets absorbed more thoroughly from the stool, and the result is hard, infrequent bowel movements. Diarrhea is, in some sense, the opposite of what the underlying disease process does.

Medications That Trigger Diarrhea

If the disease itself tends to slow the gut, why do some Parkinson’s patients develop diarrhea? In many cases, the answer is their medication. Levodopa, the most widely used Parkinson’s drug, is typically combined with another compound that prevents the body from breaking down levodopa before it reaches the brain. One of these companion drugs, benserazide, has been linked to a specific form of gut inflammation called microscopic colitis, which causes watery diarrhea.

In a case series of 21 patients taking levodopa combined with benserazide, every single one saw their diarrhea resolve when they stopped the medication. When 14 of those patients were switched to a different formulation using carbidopa instead, diarrhea went away in nine but came back in five, suggesting that in some people the problem lies with levodopa itself rather than solely with benserazide. Of the seven patients who underwent colonoscopy and biopsy, five had confirmed microscopic colitis.⁵ This reaction appears to be underdiagnosed. Patients and clinicians often assume that any gut symptom in Parkinson’s is caused by the disease rather than by its treatment, so diarrhea linked to levodopa formulations may go unrecognized for months or years.

Another Parkinson’s medication, entacapone (a COMT inhibitor that extends the effectiveness of levodopa), is well known for causing diarrhea as a side effect. Research in animal models shows that entacapone increases chloride ion secretion in the colon, which draws water into the bowel, the basic mechanism behind secretory diarrhea.⁶ This is one of the most common reasons Parkinson’s patients discontinue entacapone.

Small Intestinal Bacterial Overgrowth

When the gut slows down, bacteria that normally live in the large intestine can creep upstream and colonize the small intestine, a condition called small intestinal bacterial overgrowth, or SIBO. SIBO causes gas, bloating, abdominal discomfort, and, in many cases, diarrhea. It is far more common in Parkinson’s patients than in the general population. One study found that about a quarter of Parkinson’s patients tested positive for SIBO.⁷ Another found the rate was even higher, with over half of Parkinson’s patients affected compared to fewer than one in ten controls.⁸

The prevalence numbers vary widely between studies, likely because of differences in testing methods. But the direction is consistent: Parkinson’s disease creates conditions that favor bacterial overgrowth, and bacterial overgrowth produces symptoms including diarrhea. SIBO is also clinically important for another reason. Research has found that Parkinson’s patients with SIBO tend to have worse motor scores when they are on medication, possibly because the excess bacteria interfere with the absorption of levodopa in the small intestine.⁷ That means SIBO is not just a digestive nuisance; it can undermine the core treatment for movement symptoms.

Overflow Diarrhea From Severe Constipation

One of the more confusing scenarios for patients is “overflow diarrhea,” where severe constipation actually presents as loose stools. When the lower bowel becomes impacted with hard stool, liquid material from higher up can seep around the blockage and come out as watery diarrhea. From the patient’s perspective, it looks like diarrhea and feels like diarrhea, but the underlying problem is the exact opposite. This is not unique to Parkinson’s, but it is especially relevant here because the disease so reliably causes severe constipation. A person with Parkinson’s who develops sudden diarrhea after a period of worsening constipation should consider overflow as a possible explanation, because the treatments for the two conditions are completely different.

Gut Barrier Breakdown and Microbiome Shifts

Research into the gut environment of Parkinson’s patients has revealed changes that go well beyond slowed motility. One consistent finding is a reduction in bacteria that produce short-chain fatty acids, especially butyrate. Stool samples from Parkinson’s patients show lower levels of these compounds alongside higher levels of calprotectin, a marker of intestinal inflammation.⁹ Multiple studies have confirmed that the shift in gut bacteria composition in Parkinson’s patients tends to favor species associated with inflammation while depleting the species that help maintain a healthy gut lining.¹⁰

Butyrate plays a specific protective role: it helps tighten the junctions between cells lining the intestine. When butyrate drops, those junctions loosen, and the gut becomes “leaky,” allowing bacterial products to slip through into the bloodstream and provoke inflammatory responses.⁹ Research in animal models of Parkinson’s has shown that the alpha-synuclein buildup in the gut wall directly activates immune cells, compromises the intestinal barrier, and triggers inflammation, even before the brain shows signs of disease.¹¹

A leaky, inflamed gut does not always produce diarrhea on its own, but it creates conditions where diarrhea becomes more likely in response to dietary triggers, infections, or medications. It also increases the chances that bacterial overgrowth or food sensitivities will provoke loose stools. For many Parkinson’s patients, these microbiome and barrier changes form a background of gut vulnerability that makes any additional insult more likely to tip the balance toward diarrhea.

Gut Problems Can Precede the Diagnosis by Years

One of the more striking findings in recent Parkinson’s research is that gut dysfunction often appears long before tremor or stiffness. According to one influential hypothesis, the disease may actually begin in the gut and then travel to the brain via the vagus nerve. This idea is supported by the observation that constipation and other digestive complaints are found in virtually all confirmed Parkinson’s patients and often predate motor symptoms by a decade or more.¹² Consensus reviews have noted that gut symptoms may start years before the onset of motor symptoms and significantly impair quality of life throughout the disease course.¹³

This timeline is relevant for anyone wondering about diarrhea because it means a person with early, undiagnosed Parkinson’s might experience gut symptoms, including altered bowel habits, without connecting them to the neurological disease developing in the background. By the time motor symptoms emerge and a diagnosis is made, gut symptoms have often been present so long that they feel like a separate issue.

The Connection Between Inflammatory Bowel Disease and Parkinson’s

Researchers have noticed an unexpected overlap between Parkinson’s and inflammatory bowel disease. People with IBD, which includes Crohn’s disease and ulcerative colitis, have a higher risk of developing Parkinson’s compared to people without IBD.¹⁴ Genetic studies have found weak but statistically significant overlaps in the genes associated with both Crohn’s disease and Parkinson’s, as well as between ulcerative colitis and Parkinson’s.¹⁵

This does not mean that IBD causes Parkinson’s or vice versa. But it suggests shared biological pathways involving gut inflammation, immune activation, and possibly alpha-synuclein. For a patient who has both conditions, diarrhea is far more likely to be a prominent symptom. Even for those without a formal IBD diagnosis, the chronic low-grade gut inflammation observed in Parkinson’s may share mechanisms with IBD, which could help explain why some patients develop bowel symptoms that look more like inflammatory disease than simple motility slowing.

Pancreatic Insufficiency as a Rare Contributor

An emerging but still poorly understood possibility is that Parkinson’s may sometimes affect the pancreas’s ability to produce digestive enzymes. A published case report describes a 60-year-old man with Parkinson’s who had persistent abdominal pain and was found to have severely low levels of fecal elastase, indicating pancreatic exocrine insufficiency.¹⁶ Pancreatic exocrine insufficiency causes fatty, loose stools because the body cannot properly break down fats. Whether Parkinson’s is directly responsible for this or the two conditions merely coincided in this patient is unclear, but the case raises the question of whether the autonomic nerve damage in Parkinson’s can extend to the pancreas. If so, it would represent yet another mechanism for diarrhea that clinicians should consider.

Deep Brain Stimulation and Gut Improvement

Deep brain stimulation, or DBS, is a surgical treatment for Parkinson’s in which electrodes are placed in a specific brain region, usually the subthalamic nucleus, and deliver continuous electrical pulses. It is primarily used to control motor symptoms, but it also appears to have effects on the gut. A meta-analysis found that DBS led to a meaningful reduction in gastrointestinal symptom scores, with improvements sustained at both six months and twelve months after the procedure.¹⁷ Separate research found that after DBS, the proportion of patients with abnormal stomach electrical activity decreased, and the most dramatic improvement was in difficulty with defecation, although constipation and swallowing problems also improved.¹⁸

The DBS findings are interesting because they suggest that at least some of the gut dysfunction in Parkinson’s is being driven from the brain rather than purely from local nerve damage in the gut wall. If stimulating a brain region can improve gastric motility and reduce gut symptoms, then the central nervous system is still playing an active role in shaping how the gut behaves, even late in the disease. For patients whose diarrhea is driven by motility disturbances or gastroparesis, DBS might offer indirect relief, though the evidence is still preliminary and most of the documented improvement has been in constipation and stomach emptying rather than diarrhea specifically.

Sorting Out the Cause in Practice

For someone with Parkinson’s who develops diarrhea, the diagnostic challenge is figuring out which of several possible causes is responsible. The checklist is worth walking through with a doctor:

• Medications: Levodopa formulations, especially those containing benserazide, can cause microscopic colitis. Entacapone is a well-known offender. Dopamine agonists and other Parkinson’s drugs can also disturb the gut. A trial of stopping or switching the suspected medication is often the most informative test.
• SIBO: Breath testing can identify bacterial overgrowth. Treatment typically involves a course of antibiotics, sometimes followed by a prokinetic agent to keep the gut moving and prevent recurrence.
• Overflow: If severe constipation has been present, a plain abdominal X-ray or physical exam can reveal whether impacted stool is the underlying problem.
• Concurrent conditions: Celiac disease, IBD, lactose intolerance, and other common causes of diarrhea do not disappear just because someone has Parkinson’s. These need to be considered and tested for on their own merits.
• Pancreatic insufficiency: A fecal elastase test can screen for this, though it remains a rare finding.

The frustrating reality is that robust clinical trial data to guide the diagnosis and treatment of gut dysfunction in Parkinson’s is still lacking. Clinicians are often working from general gastroenterology principles rather than Parkinson’s-specific evidence. Patients who bring up diarrhea with their neurologist may find the conversation steered back toward constipation, which is far more familiar terrain. Being specific about the timing, character, and relationship to medications can help ensure the symptom gets properly investigated rather than dismissed as an oddity in a disease known for slowing the gut down.