Is colonoscopy prep dangerous? Barbara O'Neill's claims fact-checked

Claim 1: "The drink that you drink before the colonoscopy just about would give you irritable bowel."

Evidence check: No scientific evidence is mentioned; the claim is made without data on IBS diagnoses or long‑term outcomes after bowel prep.

Colonoscopy is widely regarded as the reference-standard test for examining the colon and can help diagnose many colorectal conditions. It can also help prevent colorectal cancer by finding and removing certain precancerous polyps (source).

To fully understand this claim, we need to understand the nature of disorders affecting the bowel. The phrase "irritable bowel" likely refers to Irritable Bowel Syndrome (IBS). IBS is a chronic functional bowel disorder defined by recurrent abdominal pain and altered bowel habits, with symptoms lasting for days, weeks or months at a time (source); it is also known as a multifactorial disorder (source), meaning that it is associated with a mix of physical, genetic and environmental triggers like stress. Transient diarrhoea and cramps from a single laxative do not meet this definition. Inflammatory bowel disease (IBD), which includes Crohn's disease and ulcerative colitis, is different: it involves chronic inflammation of the bowel.

Bowel preparation solutions used before colonoscopy are laxative solutions (commonly PEG-based or sulfate-based) designed to clear stool from the colon via diarrhoea (source, source). Effective preparation is essential to ensure the efficacy of colonoscopy, which has been shown to decrease incidence and mortality from colorectal cancer (source). Clinical information for patients commonly describes typical side effects as temporary gastrointestinal symptoms: diarrhoea, abdominal cramping, bloating, nausea and sometimes vomiting or fatigue, largely due to fluid shifts and the rapid bowel movements they induce (source). These symptoms usually resolve soon after the prep is finished.

Research directly examining the effects of bowel cleansing on the gut microbiota shows that it can temporarily change the amount and composition of gut bacteria. Studies generally suggest that the microbiota largely returns towards baseline within days to weeks; the duration and clinical relevance of these changes are still being studied (source, source, source).

While prep can temporarily worsen symptoms in those with IBD (source), and can be a source of dread for those who suffer from IBS-symptoms, the claim here is much larger in scope, implying that this could be a common complication or typical outcome. This is not supported by evidence.

Bottom line: The suggestion that a bowel prep drink may cause IBS misrepresents both the nature of the condition and the documented effects of bowel prep. More importantly, the claim overstates the risk of a routine, well-studied procedure that health professionals recommend based on a careful understanding of its risks and benefits, giving a skewed picture that undermines the importance of colonoscopies in detecting and preventing serious conditions like colorectal cancer.

Claim 2: "It is so caustic and irritating to the lining of the gut" (referring to colonoscopy prep drink)

Evidence check: The post uses strong emotive language ("so caustic") but does not cite pharmacological data or safety studies on bowel prep solutions. The claim is not supported by evidence.

The claim implies that the preparation drink administered prior to colonoscopies may burn and thus cause significant damage to the gut lining. Bowel preparations are regulated laxative medicines. Many commonly used regimens are based on polyethylene glycol (PEG) with electrolytes, while others use different validated ingredients. PEG-electrolyte solutions are iso-osmotic, meaning they are designed to minimise major fluid and electrolyte shifts while clearing the bowel, and are generally recommended for people with active IBD (source, source, source, source). This is precisely because other agents may cause mucosal abnormalities, which could mimic inflammatory bowel disease and thus make diagnosis more difficult (source). The most suitable preparation depends on the patient's health, medicines and risk factors.

Patient-facing and prescribing information for these products explains that their most common adverse effects are gastrointestinal discomfort and fluid/electrolyte shifts, not corrosive damage. Reviews and guideline documents note that bowel prep and colonoscopy can, in uncommon cases, be associated with mucosal inflammation or ischemic colitis, particularly in susceptible individuals (e.g. older patients, those with vascular disease or severe constipation) (source, source). However, these events are rare, recognised as adverse reactions, and managed within standard risk–benefit frameworks; they do not justify describing all prep drinks as generally "caustic" to the gut lining (source).

Bottom line: Using the phrase "so caustic" overstates the risk profile of these solutions, which have not been shown to damage the structure of the gut lining. Effective preparation is essential, and ongoing research into the safety and tolerability of prep solutions helps to ensure the process is both effective and appropriately managed for patients.

Final take-away

This post relies on emotive, absolute language to discourage colonoscopy, using phrases like "so caustic" that appeal to fear, rather than rely on evidence, to imply that bowel prep drinks could cause more issues than they may resolve. Claims like this are misleading not only because of what they emphasise (common fears and exaggerated risk), but because of what they leave out: the large body of clinical research and guideline evidence establishing the safety of both prep solutions and colonoscopy itself. These are not niche or untested procedures, but are established, extensively studied components available for detecting and preventing colorectal cancer (source). By omitting this context, the post turns the real but temporary discomfort of bowel preparation into a sweeping warning against a widely recommended, well-evidenced procedure.

The people most likely to be influenced by this kind of message are those already anxious about medical procedures, or those trying to manage gut health issues. If they accept the claim at face value, they may postpone or refuse a colonoscopy that has been advised to investigate bleeding, anaemia, or other potential signs of colorectal cancer or serious disease. In many cases this could mean delayed diagnosis and treatment, with real consequences for long-term health. Rather than avoiding colonoscopy because of alarming online claims, patients should discuss their individual risks, benefits and prep options with qualified healthcare professionals. As with all medical procedures, colonoscopies are not risk free, and it is important that patients feel that they have been clearly communicated both the benefits and risks associated with the procedure, in accordance with the ESGE guidance.

We have contacted Barbara O'Neill and are awaiting a response.