Diabetes and bowel preparation
In short
A coloscopy with full bowel preparation means a day or more without solid food. For people with diabetes, that changes the work the medications are doing — sometimes a great deal. Most diabetes medications need adjusting around the test. The right adjustment depends on which drug, which dose, and your usual pattern. Do not change your diabetes regimen on your own. Call the clinician who manages your diabetes for specific instructions.
What this page covers
How fasting and clear-liquid days interact with the major classes of diabetes medication, what to watch for during prep, and how to plan with the clinician who manages your diabetes care.
- Why diabetes care needs deliberate adjustment around prep
- Insulin — what generally changes
- Oral medications and non-insulin injectables
- Continuous glucose monitors and pumps
- Hypoglycaemia signs and what to do
- Type 1 versus type 2 — the differences that matter
Why this needs a deliberate plan
Bowel preparation typically means clear liquids only for at least one day, plus a fasting period before the procedure. The osmotic prep itself draws fluid into the bowel, which combines with reduced food intake to lower blood glucose in many people. Some patients on tight regimens see significant drops; some on looser regimens see little change; people who use sugar-containing clear fluids (such as full-sugar electrolyte drinks or some clear sweetened drinks) can see glucose rise.
The other reason for deliberate planning is that several classes of diabetes medication carry specific periprocedural concerns that go beyond glucose levels. SGLT2 inhibitors can predispose to euglycaemic diabetic ketoacidosis when food intake is reduced. Sulfonylureas raise hypoglycaemia risk during fasting. Metformin is generally continued in modern guidance but is often held when contrast imaging is planned (not relevant for coloscopy itself). GLP-1 receptor agonists slow gastric emptying — a separate consideration covered on a dedicated page.
Insulin — what generally changes
Insulin regimens are highly individual. Long-acting (basal) insulin is usually continued at a reduced dose during clear-liquid days; rapid-acting (bolus or mealtime) insulin given for meals is usually held when no meals are being eaten, with corrections still given for high readings. The exact percentages and the precise timing of the day-of-procedure dose come from your endocrinologist, primary care clinician, or diabetes educator.
People who use a multiple-dose insulin regimen will often be told to check glucose more frequently — every two to four hours during the prep day, and more often if symptomatic. People who use an insulin pump may be advised to lower their basal rate or use a temporary basal pattern. The pump should not be removed without instructions; the team in the procedure room will know how to manage it on the day, and many units are now familiar with leaving pumps in place during a coloscopy.
Sliding-scale corrections for hyperglycaemia are usually continued, with target ranges that reflect the situation — a slightly higher acceptable range during prep is often safer than chasing tight control while not eating.
Oral medications and non-insulin injectables
The general direction across major society guidance is summarised below. This is illustrative — the specific plan for you should come from your prescribing clinician.
| Class | Examples | Typical periprocedural pattern |
|---|---|---|
| Biguanides | Metformin | Often continued; sometimes held the day of for routine surgery, less commonly for coloscopy |
| Sulfonylureas | Glipizide, glimepiride, gliclazide | Commonly held during prep day and morning of, because of hypoglycaemia risk while fasting |
| DPP-4 inhibitors | Sitagliptin, linagliptin | Generally continued or held the day of, depending on regimen |
| SGLT2 inhibitors | Empagliflozin, dapagliflozin, canagliflozin | Commonly held for several days before the procedure to reduce risk of euglycaemic ketoacidosis |
| Thiazolidinediones | Pioglitazone | Generally continued |
| GLP-1 receptor agonists | Semaglutide, dulaglutide, liraglutide, tirzepatide | See the GLP-1 medications page; individualised approach |
| Insulin | Basal and rapid-acting | Reduced basal, held mealtime doses while not eating, corrections continued |
Again — the table orients you for the conversation. It is not a prescription.
What to use for clear liquids
Most clear-liquid lists allow broth, water, plain tea or coffee without milk, and clear juices without pulp (apple juice, white grape juice). Sugar-free options are often emphasised for diabetes. Whether you can use full-sugar electrolyte drinks depends on your glucose pattern — some people need the carbohydrate to avoid hypoglycaemia during prep; others run high on these drinks. Diet versions are generally available and acceptable for prep purposes.
Avoid red, blue, and purple drinks — they can be confused with blood at the time of the procedure. See clear liquids, defined.
Continuous glucose monitors and insulin pumps
Modern continuous glucose monitors can be left in place through the procedure and are useful for the early part of recovery. Tell the unit you are wearing one. The procedure room equipment generally does not interfere, and the data you generate during prep is genuinely useful information to share with your diabetes team afterwards.
Insulin pumps can usually remain on during the test. The team will know about it from the pre-procedure call. Do not remove a pump unless your prescribing clinician tells you to. If you use a closed-loop system with automated dosing, your educator may put you in manual mode for the prep day so you have predictable basal coverage during a period of reduced eating.
Hypoglycaemia during prep
Low blood sugar is the most common diabetes problem during prep. Symptoms include shakiness, sweating, sudden hunger, palpitations, irritability, blurred vision, confusion, or feeling weak. Treat early.
Approved fast-acting clear-liquid options for treating low glucose during prep include glucose tablets (chew and dissolve), a small amount of clear sugar-containing drink (a small glass of apple juice, for instance), or glucose gel. After treating, recheck glucose in fifteen minutes. If symptoms or numbers do not improve, repeat. If you cannot keep liquids down or your level keeps falling, call your diabetes team or the unit. Severe hypoglycaemia with confusion or unresponsiveness is a medical emergency.
Type 1 diabetes — the differences that matter
People with type 1 diabetes need basal insulin coverage at all times, even when not eating. Stopping basal insulin entirely during prep is dangerous and can produce diabetic ketoacidosis within hours. The plan involves reducing rather than stopping basal insulin, holding mealtime insulin while not eating, continuing corrections, checking glucose and ketones if glucose runs high, and bringing supplies (insulin, glucose, ketone strips) with you to the procedure.
Type 1 patients should not navigate prep alone. Call your endocrinology team well in advance — at least one to two weeks before the test — to get a written plan. If you use an insulin pump, ask whether they want a temporary basal pattern in place for the prep day.
What to ask your clinician
- What does my plan look like for each medication during the day before the test, the morning of the test, and the rest of the procedure day?
- What target range should I aim for during prep, and what should make me call you?
- Do I need to hold my SGLT2 inhibitor, and for how many days before?
- How should I adjust basal insulin? Mealtime insulin?
- Can I eat or drink something with carbohydrate if my glucose drops?
- What do I do if I have nausea, vomiting, or cannot keep prep down — does that change the plan?
- Should I bring snacks for after the procedure to eat as soon as I am cleared?
- If I use a pump or CGM, what does the unit need to know?
Common worries, briefly addressed
Will my glucose run wild during prep?
Some movement is expected. With a written plan from your team and frequent checks, most people stay within an acceptable working range during prep. A few brief excursions in either direction are not unusual; sustained extremes warrant a call.
What if I have a hypoglycaemic episode the morning of the test?
Treat it. Do not arrive for a coloscopy with severe low glucose — tell the unit. They will probably want a current reading and possibly another point-of-care check before sedation. The procedure can be slightly delayed without changing its outcome.
Should I bring my glucose meter?
Yes. Also bring snacks for afterwards, your usual medications in their original containers (unless told otherwise), and any insulin or supplies you might need during a longer-than-expected wait.
Can I have my pump removed during the procedure?
It is usually unnecessary. Tell the unit in advance so a plan is in place; some units prefer to leave it on, others may want it placed in a particular site that won't be near the procedure field.
What about ketosis? Should I check ketones?
People with type 1 diabetes are generally taught to check ketones when glucose is high or when ill. Bring strips and check if symptoms develop or glucose runs high during prep. SGLT2 inhibitor users should be especially aware that ketoacidosis can occur with normal-looking glucose readings.
Sources
- American Diabetes Association — Standards of Care, sections on inpatient and perioperative care
- Diabetes UK — guidance for managing diabetes around procedures
- Joint British Diabetes Societies — guidelines on management of adults with diabetes undergoing surgery and elective procedures
- European Association for the Study of Diabetes — periprocedural management consensus
- American Society of Anesthesiologists — perioperative considerations for SGLT2 inhibitors and GLP-1 receptor agonists