What Can Go Wrong During a Colonoscopy?

Colonoscopy is one of the safest procedures in medicine relative to how commonly it is performed, but “safe” is not the same as “risk-free.” Large studies tracking tens of thousands of procedures report perforation rates between roughly 0.005% and 0.085%, and post-procedure bleeding in up to about 0.7% of cases, with the higher end of that range involving polyp removal rather than simple screening.¹ Those numbers are reassuring, but the range of things that can go wrong is broader and more varied than most people realize before they consent to the procedure.

Perforation of the Colon Wall

The complication that worries gastroenterologists most is perforation, a tear that goes all the way through the colon wall and allows bowel contents to leak into the abdominal cavity. This can happen in a few ways. Mechanical perforation occurs when the tip of the colonoscope pushes too hard against the intestinal wall. Experimental work measuring how much force is needed to puncture the colon found that the threshold depends on the shape of the scope tip and the angle of contact, but the forces that colonoscopists typically exert during a procedure can overlap with the force needed to cause a tear, especially when the scope presses against a thinner part of the bowel.² In plain terms, there is not always a generous margin of safety between “enough pressure to advance the scope” and “too much pressure.”

A second mechanism is barotrauma, where overinflation of the colon with gas can tear the wall from the inside. This has been documented even with carbon dioxide insufflation, which is generally considered gentler than room air.³ A third route is thermal injury during polyp removal, where electrical current used to cut or cauterize tissue can transmit heat through the colon wall and damage adjacent structures, including the small intestine on the other side.⁴

A full-thickness tear is sometimes spotted immediately during the procedure, but recognition can be delayed by up to three days.⁵ That delay is important because a perforation left untreated leads to peritonitis and potential sepsis. Anyone who develops worsening abdominal pain, fever, or rigidity in the days after a colonoscopy should treat it as a medical emergency, even if the procedure itself seemed to go smoothly.

Bleeding After Polyp Removal

The most common complication overall is bleeding, and it is overwhelmingly linked to polypectomy rather than to simple diagnostic colonoscopy. In a screening study from the Valencian Community, about two thirds of severe complications occurred during therapeutic procedures where a polyp or cancer was removed, compared to roughly one third during purely diagnostic exams.⁶

Bleeding after polypectomy can be immediate, which the endoscopist usually handles on the spot, or delayed by days. A study tracking over 15,000 polypectomies found that delayed bleeding typically showed up around four days later, with the vast majority appearing within about a week, though in rare cases it surfaced as late as 16 days after the procedure.⁷ Several factors make delayed bleeding more likely:

Polyp size: Polyps larger than 10 mm carry roughly four to five times the bleeding risk of smaller ones.⁷
Polyp shape: Pedunculated polyps, the kind that hang on a stalk, bleed at roughly three and a half times the rate of flat sessile polyps.⁸
Location: Polyps in the right side of the colon carry about three times the risk of those on the left.⁸
Body weight: Higher BMI was independently associated with roughly a threefold to fourfold increase in delayed bleeding risk.⁸

Blood-thinning medications add another layer of concern. An analysis of over 1,600 polypectomies found that warfarin use was associated with a significant increase in post-polypectomy bleeding, while antiplatelet agents alone were not.⁹ Deciding whether to pause anticoagulants before a colonoscopy involves weighing the bleeding risk against the risk of a blood clot if the medication is stopped.¹⁰ This is worth a direct conversation with your gastroenterologist and cardiologist or primary care doctor well before the procedure date.

Post-Polypectomy Electrocoagulation Syndrome

This one is a lesser-known complication, even among people in the medical field. When electrical cautery is used to remove a polyp, the heat can create a transmural burn, meaning it damages the full thickness of the colon wall without actually perforating it. The result looks and feels a lot like perforation: abdominal pain, fever, elevated white blood cell count, and localized tenderness, all appearing within a day or two of the procedure.¹¹

The good news is that it usually follows a benign course and can be managed conservatively with bowel rest and antibiotics, without surgery. The bad news is that the initial presentation is alarming and often triggers CT scans and hospital admission while doctors rule out a true perforation. In rare cases, though, post-polypectomy syndrome can progress to sepsis and become life-threatening, so it should never be brushed off as “just” discomfort after a procedure.¹²

Sedation Reactions

Most colonoscopies in the United States involve some form of sedation, usually propofol or a combination of a benzodiazepine and an opioid. The primary sedation-related risks are drops in blood pressure, a slowed heart rate, and respiratory depression, where breathing becomes too shallow or stops momentarily. Meta-analyses comparing propofol to traditional sedation regimens have found broadly similar rates of these events. One large Cochrane review found that propofol likely makes little difference in respiratory events requiring intervention compared to standard agents.¹³ A separate meta-analysis echoed this, finding comparable complication rates overall.¹⁴

That said, sedation responses are highly individual. Older adults, people with obesity, and those with sleep apnea or lung disease are at higher risk for airway problems during sedation. Propofol in particular has no reversal agent once administered, unlike benzodiazepines, which can be reversed with flumazenil. Unsedated colonoscopy is an option in some centers, though it is far more common in parts of Europe and Asia than in North America. The choice of sedation strategy is worth discussing with your endoscopy team, especially if you have had prior sedation complications or airway concerns.

What the Bowel Prep Can Do to You

The colonoscopy prep itself, the large-volume liquid you drink the day before to clean out the colon, is the part most patients dread. Beyond the unpleasantness, it carries real physiological risks. The main concern is electrolyte disturbance. A systematic review and meta-analysis found that sodium phosphate-based preps were associated with low potassium in about 17% of patients, compared to about 5% for polyethylene glycol-based preps.¹⁵ Low potassium can cause muscle cramps, weakness, and in severe cases, dangerous heart rhythm problems.

Beyond potassium, bowel prep agents have been associated with kidney function deterioration and other electrolyte disorders, including cases that were serious or fatal, particularly in patients who already had compromised kidney function.¹⁶ Sodium phosphate preps carry the most kidney risk and are now used far less frequently than they were a decade ago. If you have chronic kidney disease or heart failure, your doctor should prescribe a prep formulation that accounts for your fluid and electrolyte limitations. Dehydration during prep is common and often self-inflicted, since people sometimes limit fluid intake to reduce nausea, when they should actually be drinking more.

Infection From the Scope

Flexible endoscopes are among the medical devices most frequently implicated in outbreaks of healthcare-associated infections, primarily because they cannot be heat-sterilized and have internal channels that are difficult to clean.¹⁷ Bacteria can form biofilms on the inner surfaces of these channels, and these biofilms resist standard disinfection.

A prospective study that cultured colonoscopes after routine decontamination found that about 2% of supposedly patient-ready instruments still harbored viable organisms. Even more striking, when researchers tested culture-negative washes using DNA analysis, about 40% contained coliform bacterial DNA, suggesting biofilm was present even when cultures came back clean.¹⁸ In practice, clinically significant infections after colonoscopy remain rare, but the possibility is not zero, and large outbreaks linked to contaminated duodenoscopes and colonoscopes have made headlines periodically. This is a risk the patient has essentially no control over. It depends entirely on the facility’s reprocessing protocols and equipment maintenance.

Splenic Injury and Other Unusual Complications

The spleen sits near the left bend of the colon, connected to it by a ligament. When the colonoscope loops or when external pressure is applied to the left upper abdomen to help guide the scope, that ligament can pull on the spleen, causing tears, hematomas, or in severe cases, rupture. A review of 66 reported cases found that the majority of splenic injuries occurred during otherwise uneventful colonoscopies, meaning the endoscopist had no particular difficulty and had no reason to suspect a problem.¹⁹ The mortality rate in this case series was about 4.5%, and over half of patients required surgery to remove the spleen.

Symptoms usually appear within 24 hours, typically as left-sided abdominal pain that can radiate to the shoulder.²⁰ Delayed presentations, showing up more than 48 hours later, tend to be subtler and harder to diagnose.²⁰ This is rare enough that many gastroenterologists will go their entire career without seeing a case, but it deserves mention because left upper abdominal pain after a colonoscopy should prompt consideration of splenic injury, and some patients may not connect the symptom to the procedure.

Even more unusual is colonoscopy-triggered appendicitis. A documented case involved a 48-year-old woman with no prior symptoms who developed right lower quadrant pain the evening after an uneventful screening colonoscopy. Surgical exploration confirmed acute appendicitis.²¹ The presumed mechanism is that scope manipulation or insufflation near the appendiceal orifice can trigger inflammation or obstruction. Cases like this are rare enough to be published individually as case reports, which tells you something about how uncommon they are.

When the Scope Cannot Reach the End

An incomplete colonoscopy is not a complication in the traditional sense because it does not injure you, but it is a failure of the procedure’s purpose. If the scope does not reach the cecum, the very beginning of the colon, any polyps or cancers in the unexamined segments are missed. One study analyzing reasons for incomplete exams found that the most common was a tortuous colon, accounting for about 30% of failures. Pain or inadequate sedation caused about 16%, poor preparation about 11%, stricturing disease about 10%, and obstructing masses or diverticular disease each contributed smaller shares.²² CT colonography work has confirmed that anatomic features like colonic elongation and tortuosity are strong predictors of the scope failing to reach the end.²³

If your colonoscopy is incomplete, the next step is typically a CT colonography (virtual colonoscopy) or a repeat attempt, sometimes with an experienced operator or a different scope. Some people simply have colons that are unusually long or loopy, and this is not something you can predict or prevent. What you can control is the quality of your prep, since poor preparation is one of the preventable causes of failure, and adequate sedation, which your anesthesia team can adjust.

Who Faces the Highest Risk

Risk is not evenly distributed. Two factors stand out consistently across the literature. The first is age: patients over 75 have substantially higher perforation risk. One study found the odds of perforation were about six times higher in this group compared to younger patients.²⁴ The colon wall thins with age, diverticular disease becomes more prevalent, and the tissue becomes less forgiving of the pressures involved.

The second major factor is whether the procedure is diagnostic or therapeutic. Colonoscopies that involve polyp removal carry higher complication rates than purely diagnostic ones. In the same study, therapeutic endoscopy nearly tripled the odds of perforation.²⁴ Patients with inflammatory bowel disease also face increased risks of post-colonoscopy complications.¹ None of this means older adults or people with IBD should avoid colonoscopy, as the benefits of cancer detection generally outweigh these elevated risks. But it does mean the informed consent conversation should be more detailed for these groups, and alternative screening approaches like stool-based tests may deserve more weight in the decision.

Gas, Bloating, and Post-Procedure Discomfort

Not every problem after a colonoscopy qualifies as a complication. Bloating, cramping, and gassiness are extremely common and are caused by the air or gas used to inflate the colon during the exam. The colon needs to be distended for the camera to see the walls clearly, and that gas has to go somewhere afterward.

Carbon dioxide insufflation has replaced room air at many centers because CO₂ is absorbed through the bowel wall and eliminated through the lungs much faster than nitrogen-rich room air. An early randomized trial found dramatic differences: 45% of patients insufflated with room air had pain at one hour versus 7% in the CO₂ group, and residual colonic distension was almost nonexistent with CO₂ compared to most patients having visible bloating on X-ray in the air group.²⁵ A trial in pediatric patients similarly found significantly less bloating with CO₂, along with a reduced need for pain medication during the procedure.²⁶ If post-procedure comfort matters to you, it is worth asking your endoscopy center whether they use CO₂.

What AI-Assisted Colonoscopy Adds to the Picture

Computer-aided polyp detection, where an artificial intelligence system highlights suspicious spots on the video feed in real time, is increasingly common in endoscopy suites. The technology catches small polyps that the human eye might miss, which sounds like a straightforward improvement. But increased detection creates its own form of burden. A modeling study estimated that using AI-assisted detection during screening colonoscopy would lead to an additional 64 people per 1,000 being sent for surveillance colonoscopy, basically a follow-up exam within a few years, because more small adenomas were found and classified as needing monitoring.²⁷

The reassuring part is that the simulation did not find a clinically relevant increase in direct patient harms like perforation or bleeding, mostly because the additional polyps detected tend to be small and low-risk to remove.²⁷ The harm, if any, is more subtle: more anxiety, more follow-up appointments, more time spent in medical settings, and the accumulated small risks of additional procedures over a lifetime. Whether that tradeoff is worth it depends on your values and on how much cancer prevention you want to buy at the cost of increased medical surveillance. It is the kind of question that colonoscopy screening will grapple with more as AI adoption spreads.

1
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3
PubMed Central. “Cat Scratch Colon” and Cecal Barotrauma perforation during colonoscopy using CO2 insufflation
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PubMed Central. Small bowel perforation caused by thermal injury during colonic polypectomy: A case report and literature review
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PubMed. Risk of colonoscopic polypectomy bleeding with anticoagulants and antiplatelet agents: analysis of 1657 cases
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PubMed Central. Post-polypectomy electrocoagulation syndrome: a rare cause of acute abdominal pain
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PubMed Central. Postpolypectomy syndrome without abdominal pain led to sepsis/septic shock and gastrointestinal bleeding: A case report
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PubMed. Propofol for sedation during colonoscopy
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PubMed. Effect and safety of propofol for sedation during colonoscopy: A meta-analysis
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PubMed Central. Electrolyte disturbances after bowel preparation for colonoscopy: Systematic review and meta‐analysis
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PubMed Central. Colonoscopy preparation-induced disorders in renal function and electrolytes
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PubMed Central. Transmission of infection by flexible gastrointestinal endoscopy and bronchoscopy
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PubMed. A prospective study of the efficacy of routine decontamination for gastrointestinal endoscopes and the risk factors for failure
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PubMed. Splenic injury in colonoscopy: a review
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PubMed Central. Splenic rupture following colonoscopy: Case report and literature review
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PubMed Central. Acute appendicitis caused by colonoscopy
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PubMed Central. Incomplete colonoscopy: maximizing completion rates of gastroenterologists
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PubMed. Anatomic factors predictive of incomplete colonoscopy based on findings at CT colonography
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BMC Gastroenterology. What are the risk factors of colonoscopic perforation?
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PubMed. Minimizing postcolonoscopy abdominal pain by using CO(2) insufflation: a prospective, randomized, double blind, controlled trial evaluating a new commercially available CO(2) delivery system
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PubMed Central. Carbon dioxide versus room air for colonoscopy in deeply sedated pediatric patients: a randomized controlled trial
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PubMed Central. Benefits, burden, and harms of computer aided polyp detection with artificial intelligence in colorectal cancer screening: microsimulation modelling study