Chills during colonoscopy prep are a recognized side effect that many people experience, and in most cases they are not a sign that anything is going wrong. At least one clinical trial explicitly listed chills among the commonly reported adverse events during bowel preparation for colonoscopy, alongside nausea, vomiting, and headache.1Gastrointestinal Endoscopy. Split-dose administration of a dual-action, low-volume bowel cleanser for colonoscopy: the SEE CLEAR I study The underlying reasons are more interesting than most people realize, and understanding them can help you figure out whether your chills are ordinary discomfort or something worth calling your doctor about.
Why Colonoscopy Prep Causes Chills in the First Place
Colonoscopy prep works by flushing the contents of your colon so the doctor can see the lining clearly. To accomplish that, most prep solutions are either osmotic laxatives, which pull water from your body into the intestine, or high-volume solutions that physically wash out the colon. Either way, your body loses a significant amount of fluid in a short window. That rapid fluid loss is the primary reason people get chills.
When you lose fluid quickly through diarrhea, your blood volume drops. Your body responds by constricting blood vessels near the skin to keep blood flowing to vital organs. That vasoconstriction is the same reflex that makes you feel cold when you step outside on a winter day: less warm blood reaches your skin’s surface, and you start shivering. During prep, the effect can be surprisingly intense because the fluid loss happens over just a few hours rather than gradually over a day.
There is also a straightforward temperature component. Drinking a large volume of cool or room-temperature liquid can lower your core body temperature slightly, especially if you are already losing body heat through frequent trips to the bathroom. Some people notice that their chills come in waves, often coinciding with another round of diarrhea, which makes sense given the burst of fluid and heat leaving the body each time.
The Role of Electrolyte Shifts
Beyond simple fluid loss, the prep solution itself can shift the balance of electrolytes in your blood. A systematic review and meta-analysis examining electrolyte disturbances after bowel preparation found that sodium phosphate-based preps are highly osmotic and result in fluid shifts from the bloodstream into the gastrointestinal tract.2PubMed Central. Electrolyte disturbances after bowel preparation for colonoscopy: Systematic review and meta‐analysis Those fluid shifts do not just dehydrate you; they also drag electrolytes along with them.
Potassium is one of the electrolytes most commonly affected. That same meta-analysis found that low potassium (hypokalemia) occurred in roughly 17% of patients using sodium phosphate preps compared with about 5% of those using polyethylene glycol (PEG) preps.3PubMed Central. Electrolyte disturbances after bowel preparation for colonoscopy: Systematic review and meta‐analysis – Section: RESULTS Low calcium levels were also observed in about 16% of the sodium phosphate group versus 8% in the PEG group. These electrolyte dips can cause symptoms ranging from mild muscle weakness and nausea to, in rare and severe cases, cardiac rhythm problems or seizures.3PubMed Central. Electrolyte disturbances after bowel preparation for colonoscopy: Systematic review and meta‐analysis – Section: RESULTS
Chills specifically have not been pinned down to one single electrolyte change in most studies, but the general picture is clear: when your body’s fluid and electrolyte balance gets disrupted quickly, the autonomic nervous system reacts, and shivering and chills are part of that reaction. Most of these disturbances are mild and self-correcting once you resume eating and drinking normally after the procedure.
Does the Type of Prep Solution Matter?
People sometimes wonder whether switching to a different prep formula would spare them the chills. The answer is a qualified “maybe.” Different prep solutions do differ in their side-effect profiles, though the differences are not always as dramatic as you might hope.
One older randomized trial comparing several bowel-cleansing methods found no significant differences in the frequency or intensity of any of the eleven side effects they tracked.4PubMed. Prospective, randomized, endoscopic-blinded trial comparing precolonoscopy bowel cleansing methods That suggests that many of the uncomfortable symptoms, chills included, are at least partly inherent to the process of rapidly emptying the colon, regardless of which product you use.
That said, the electrolyte data tell a more nuanced story. Sodium phosphate preps pull harder on your body’s fluid reserves and cause more pronounced potassium and calcium drops than PEG-based preps do. If your chills are driven partly by those electrolyte shifts rather than just fluid loss, a PEG-based solution might produce somewhat milder symptoms. However, PEG preps typically require you to drink a larger total volume of liquid, which comes with its own discomfort. The trade-off is real, and the “best” prep is usually whichever one your gastroenterologist recommends based on your medical history, kidney function, and age.
Split-Dose Prep and Side Effects
Many gastroenterology practices now recommend split-dose regimens, where you drink half the prep the evening before and the other half early the morning of the procedure. This approach was originally developed to improve how clean the colon is at the time of the scope, but it may also help with side effects.
A clinical trial evaluating a split-dose, low-volume bowel cleanser reported that nausea, vomiting, headache, and chills were commonly reported adverse events.1Gastrointestinal Endoscopy. Split-dose administration of a dual-action, low-volume bowel cleanser for colonoscopy: the SEE CLEAR I study In that trial, patient-reported tolerability, including ease of consumption and taste, was significantly higher for the newer low-volume prep than for the traditional PEG plus bisacodyl combination. The logic behind split-dosing helping with chills is straightforward: by spreading the fluid loss over two sessions several hours apart, you give your body more time to compensate. You are not hit with all of the dehydration and electrolyte shifts in one punishing evening.
If you experienced severe chills with an evening-only prep in the past and are due for another colonoscopy, asking your doctor about a split-dose option is reasonable. It may not eliminate the chills entirely, but distributing the stress on your body over a longer window tends to reduce the peak intensity of most symptoms.
Practical Ways to Manage Chills During Prep
Since chills during prep are driven mainly by fluid loss, cooling of the body, and electrolyte shifts, the countermeasures are fairly intuitive:
- Stay warm: Keep a blanket nearby, wear warm socks, and consider layering up. This sounds obvious, but many people strip down to light clothing for easy bathroom access and then wonder why they are shivering.
- Drink approved clear fluids between prep doses: Your prep instructions will specify what you can and cannot drink, but most allow clear broth, sports drinks, apple juice, and plain water. Sipping warm broth between trips to the bathroom can both replace some electrolytes and physically warm you.
- Keep the room warm: Turn up the thermostat if possible. Your body is already losing heat through fluid loss; a cold room compounds the problem.
- Time your prep strategically: If you are doing a split dose, finishing the evening portion early enough to get some sleep before starting the morning dose can help your body recover partially between rounds.
Warm broth deserves a special mention because it does double duty. Chicken or vegetable broth supplies sodium, which helps your body hold onto water, and the heat directly combats the chills. Most prep instructions explicitly permit clear broth, though you should avoid anything with pulp, solids, or red or purple coloring.
When Chills Might Signal a Real Problem
Mild chills and shivering during prep are expected. But there is a point where the symptoms cross from uncomfortable but harmless to genuinely concerning. The key warning signs to watch for include:
- High fever: Chills without fever are typical during prep. Chills accompanied by a temperature above 101°F (38.3°C) could indicate an infection or a more significant systemic response. Call your doctor’s office or on-call line.
- Severe muscle cramps or spasms: Mild muscle twitching can happen with electrolyte shifts, but intense cramping in the legs, hands, or face suggests your potassium or calcium has dropped significantly.
- Heart palpitations or chest pain: Significant electrolyte disturbances can affect heart rhythm. If you feel your heart racing, skipping beats, or fluttering alongside your chills, that warrants immediate medical attention.
- Confusion or extreme dizziness: These can signal severe dehydration or dangerous electrolyte levels. If you cannot stand safely or feel mentally foggy beyond what tiredness would explain, contact your care team.
- Inability to keep fluids down: If vomiting prevents you from finishing the prep and replacing fluids, your dehydration risk climbs. Most endoscopy centers have protocols for this, and calling them early is better than pushing through and arriving severely dehydrated.
People taking diuretics, blood pressure medications, or kidney medications are at higher risk for electrolyte problems during prep. If you fall into one of those groups, your doctor likely already adjusted your prep instructions, but bring up any unusual symptoms promptly rather than assuming they will pass.
Who Gets Chills More Often
Not everyone shivers through their colonoscopy prep. Certain factors make it more likely. Smaller body mass means less thermal reserve, so lighter individuals tend to feel colder faster. Older adults are more susceptible both because their thermoregulation is less efficient and because they are more likely to be on medications that affect fluid and electrolyte balance. People who are already mildly dehydrated going into the prep, whether from not drinking enough the day before or from a chronic condition, tend to hit the shivering phase sooner.
Anxiety can play a role, too. The anticipation of an unpleasant experience activates the sympathetic nervous system, and that activation can produce chills that have nothing to do with fluid loss. If you have ever felt a shiver run through you during a stressful moment, the mechanism is similar. Some people find that their chills ease once they settle into the process and realize the prep, while unpleasant, is manageable.
Women may be more prone to feeling cold during prep than men, largely because of differences in body composition and baseline metabolic rate. This is speculative in the specific context of colonoscopy prep, since most studies do not break out chills by sex, but it aligns with the well-established observation that women report feeling cold more often in general settings.
Chills During Prep Versus Shivering After the Procedure
Some people confuse the chills they get during prep with shivering that occurs after the colonoscopy itself, but these are different phenomena with different causes. Post-procedure shivering is well-documented and usually related to sedation. Many anesthetic and sedative drugs lower your body temperature by dilating blood vessels and suppressing the shivering reflex while you are under. Once the drugs wear off, your body realizes it has cooled down and starts shivering aggressively to warm back up. The endoscopy suite itself is often kept cool for equipment and infection-control reasons, which adds to the effect.
Prep chills, by contrast, happen hours before you arrive at the procedure center and are driven by dehydration, electrolyte shifts, and the physical cooling effect of rapid fluid loss. If you shivered during the prep night but felt fine during and after the procedure, that is a different story than someone who felt fine during prep but could not stop shaking in the recovery room. Knowing the difference matters mostly because it affects what you do about it: prep chills call for warmth and fluid replacement, while post-procedure shivering resolves on its own as the sedation clears and warm blankets do the rest.
Repeat Colonoscopies and Learning From Past Preps
If you are due for a repeat colonoscopy, perhaps on the standard ten-year screening interval or sooner because of polyp surveillance, your previous experience is actually useful information. Tell your gastroenterologist what happened last time. If you had severe chills, significant nausea, or could not finish the prep, they can adjust the plan. Options include switching the type of prep solution, moving to a split-dose schedule, or prescribing anti-nausea medication to take alongside the prep.
Some people discover through trial and error that chilling the prep solution makes it easier to drink but worsens their chills, while drinking it at room temperature is less palatable but keeps them warmer. Others find that starting the prep earlier in the evening, when they are better fed and hydrated from the day, reduces the intensity of the chills compared to starting later. These small adjustments are worth experimenting with, within whatever guidelines your doctor gives you.
The experience of colonoscopy prep has genuinely improved over the past two decades. Newer low-volume formulations require you to drink less liquid overall, and split-dosing has become the standard recommendation at most major centers. Neither change eliminates side effects, but both tend to reduce their severity. If your last colonoscopy prep was years ago and you are dreading a repeat, the process you face now is likely a bit less punishing than what you remember.