by Jane Hind
(Australia)
QUESTION: I have taken metformin 500 mg, two tablets in the morning and two at night, for several years. My home readings twice weekly seem acceptable, but my HbA1c is 8.2%. Diabex XR caused nausea and diarrhea. I started insulin yesterday and still have the symptoms. I also have recurrent occult blood, anemia, and previously treated colonic angiodysplasia. Could the bleeding or bowel condition affect my diabetes treatment?
Diabex XR is extended-release metformin, not a different diabetes medicine. Nausea and diarrhea are common metformin effects, particularly after a dose or formulation change. Injected insulin does not usually cause diarrhea, although low glucose can cause weakness, sweating, nausea, or confusion. Recurrent gastrointestinal bleeding and anemia need separate urgent evaluation and can make HbA1c misleading. Contact the treating doctor promptly to review the metformin, hydration, kidney function, anemia, glucose pattern, and insulin plan.
Answer by Dr. Albana Greca, MD, MMedSc
Hi Jane,
Your symptoms may have more than one cause. The diabetes medicines, active gastrointestinal disease, blood loss, anemia, food intake, and glucose levels should be reviewed together rather than assuming that you are intolerant of every treatment.
Diabex XR contains metformin in an extended-release formulation. It releases the medicine more slowly and is often tried when standard metformin causes stomach problems, but it can still cause nausea, diarrhea, abdominal discomfort, gas, or loss of appetite.
You appear to have been taking 2,000 mg daily. The clinician should confirm the exact previous and new doses, whether Diabex XR was introduced gradually, and whether it was taken with food. Do not combine immediate-release metformin with Diabex XR unless the prescription specifically says to do so.
Insulin does not usually cause persistent diarrhea. Because it was started only yesterday, continued symptoms are more likely to be related to metformin, the bowel condition, another medicine, infection, or another gastrointestinal cause.
Insulin can cause hypoglycemia. Check glucose during nausea, weakness, sweating, trembling, drowsiness, or confusion. If glucose is below 70 mg/dL and you can swallow safely, take about 15 grams of fast-acting carbohydrate and recheck after 15 minutes.
Angiodysplasia consists of fragile abnormal blood vessels in the gastrointestinal tract that can bleed again after treatment. Chronic occult bleeding may cause iron-deficiency anemia, fatigue, weakness, shortness of breath, dizziness, and reduced exercise tolerance.
The bleeding does not usually cause metformin intolerance directly. However, active diarrhea, poor intake, dehydration, acute kidney injury, or serious illness may change whether metformin is safe. Ask the treating clinician whether metformin should be temporarily withheld under a sick-day plan; do not make that decision alone.
Checking only twice weekly may miss high glucose after meals, overnight, or on other days. A structured meter profile or continuous glucose monitor may show whether the HbA1c reflects true hyperglycemia.
Anemia can also distort HbA1c. Iron-deficiency anemia may make HbA1c appear higher than the actual average glucose, while active blood loss, transfusion, or treatment that changes red-cell production can alter it in other directions. HbA1c of 8.2% should therefore be interpreted with the blood count, iron studies, glucose records, and bleeding history.
If HbA1c and measured glucose remain clearly inconsistent, the clinician may use CGM, a structured glucose profile, fructosamine, or glycated albumin while the anemia is evaluated.
Long-term metformin can contribute to vitamin B12 deficiency, but it does not explain a positive occult-blood test. Both iron deficiency from bleeding and possible B12 deficiency should be assessed correctly.
Review our guides to metformin side effects, insulin safety, and HbA1c interpretation.
Seek urgent care for black tarry stool, visible or heavy rectal bleeding, vomiting blood, fainting, chest pain, severe breathlessness, rapid heartbeat, marked weakness, inability to keep fluids down, very little urine, or severe abdominal pain. Call emergency services for unconsciousness, seizure, inability to swallow, severe confusion, or a low glucose that does not improve after treatment.
The most important corrections are that Diabex XR is metformin and that insulin is unlikely to explain persistent diarrhea. Recurrent bleeding and anemia may explain weakness and may distort HbA1c, while limited home testing may miss high glucose. The medication plan and gastrointestinal investigation should be coordinated rather than managed separately.
Educational safety note: This answer is for general diabetes education only. It does not replace personal gastroenterology or diabetes care. Do not start, stop, combine, or change metformin, Diabex XR, insulin, iron, supplements, or other medicines without speaking with the treating healthcare team.
Last reviewed: July 2026.
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