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Medications

Metformin Side Effects: What's Normal, What to Watch For, and When to Call the Doctor

The most common metformin side effects are gastrointestinal—nausea, diarrhea, and stomach cramping—especially when starting or increasing the dose. These often improve within weeks and may ease if you take it with food or switch to extended-release. Serious effects are rare but include lactic acidosis, a medical emergency.

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What Are the Most Common Metformin Side Effects?

Most people who experience side effects notice them in the digestive system 12:

  • Diarrhea — the most frequently reported effect; typically worst in the first weeks
  • Nausea — especially if taken on an empty stomach
  • Stomach cramping or discomfort
  • Vomiting — less common than nausea
  • Metallic taste in the mouth — recognized but harmless; some people notice this, others do not
  • Decreased appetite — often a neutral or welcome effect for people managing type 2 diabetes or prediabetes

GI side effects are more common with: - Immediate-release (IR) metformin compared to extended-release (ER or XR) formulations - Higher doses - Starting at full dose rather than increasing gradually - Taking it without food

For most people, GI symptoms are worst in the first two to four weeks and then ease as the body adjusts. A gradual dose-titration plan — starting low and increasing slowly over several weeks — is a standard strategy to minimize these effects 1.

Does Metformin Affect Vitamin B12?

Long-term metformin use can reduce the absorption of vitamin B12 from the gut 3. This effect is gradual and asymptomatic at first. Low B12 can eventually cause fatigue, numbness or tingling in the hands and feet, and problems with balance or memory.

Guidelines recommend monitoring B12 levels periodically in people taking metformin long-term 1. If your clinician has not mentioned this, it is a reasonable question to raise at your next visit — B12 supplementation is straightforward and effective when a deficiency is caught early.

What Is Lactic Acidosis and How Rare Is It?

Lactic acidosis is a buildup of lactic acid in the blood. It is a rare but potentially life-threatening complication associated with metformin — most likely to occur when the kidneys are not working well, because metformin is cleared by the kidneys 2.

Risk is higher if you: - Have kidney disease or reduced kidney function (eGFR below certain thresholds leads to dose adjustment or discontinuation) - Are severely dehydrated from illness, vomiting, or diarrhea - Use alcohol heavily - Have had IV contrast dye for imaging (such as a CT scan with contrast)

For this reason, metformin is typically paused before procedures using IV contrast dye and restarted only after kidney function is confirmed stable. Make sure any radiologist or specialist knows you take metformin before such a procedure 2.

What Helps with GI Side Effects?

Most people find these strategies helpful:

  • Take it with meals — the most reliable way to reduce nausea and stomach upset
  • Start low and increase slowly — if your prescriber provided a gradual titration plan, follow it carefully
  • Switch to extended-release (ER/XR) metformin — the extended-release form releases more slowly and causes substantially less GI upset for many people 1; ask your prescriber if you are having persistent trouble with the immediate-release version
  • Be patient with the first few weeks — GI symptoms that begin with the medication often improve on their own as the body adjusts

What Lab Tests Are Relevant for People on Metformin?

Your clinician will likely monitor several values periodically 12:

  • Kidney function (creatinine and eGFR) — metformin dosing and safety depend on kidney function; reduced function increases the risk of metformin accumulation
  • Vitamin B12 — periodic monitoring allows for supplementation before a deficiency causes symptoms
  • HbA1c (glycated hemoglobin) — assesses whether metformin is effectively managing blood sugar and whether the dose is appropriate
  • Lactate — checked in an emergency setting if lactic acidosis is suspected

Common questions

How long does metformin stomach upset usually last?

For most people, GI side effects are worst in the first two to four weeks and improve as the body adjusts. If symptoms are still significantly disruptive after a month, it is worth contacting your prescriber — switching to the extended-release formulation resolves GI problems for many people.

Should I skip metformin if I am sick with vomiting or diarrhea?

This is a reasonable precaution. Severe dehydration from illness temporarily reduces kidney function, which is one of the risk factors for lactic acidosis with metformin. Many clinicians advise holding metformin and calling the office when you are too ill to keep fluids down. Ask your prescriber in advance what their guidance is so you have a clear plan before illness strikes.

Can metformin cause low blood sugar?

Metformin alone rarely causes low blood sugar (hypoglycemia) — it is one of the reasons it is often preferred as a first-line medication. However, when metformin is combined with insulin or sulfonylureas (such as glipizide or glimepiride), low blood sugar becomes a real risk. Know which other diabetes medications you take.

I have been on metformin for years. Should I be tested for B12 deficiency?

Yes, periodic B12 monitoring is recommended for people on long-term metformin. If you are not sure whether your B12 has been checked recently, it is a straightforward question to raise at your next appointment. Low B12 is easily treated with supplements, and catching it before symptoms develop is far preferable to treating symptoms after the fact.

Talk to a clinician

Nina Osei, NP — Nurse Practitioner

checkups, refills & skin. Gale can match you with a licensed clinician for a visit.

Find care →

Signs That Require Emergency Care

  • —Unusual muscle pain throughout the body, especially without a clear physical cause
  • —Trouble breathing or rapid, labored breathing
  • —Severe stomach pain
  • —Feeling cold, dizzy, or lightheaded without another explanation
  • —Irregular or very slow heartbeat
  • —Unusual weakness or fatigue that came on quickly
  • —Signs of low blood sugar (sweating, shaking, confusion) if you take other diabetes medications alongside metformin

If you experience muscle pain, difficulty breathing, severe stomach pain, or unusual weakness — especially if you are also dehydrated or ill — call 911 or go to the nearest emergency room immediately. These can be signs of lactic acidosis, which is a medical emergency.

This article provides general health education and does not constitute medical advice or a personalized assessment of your medication. Side effect profiles vary by individual. Always speak with your prescribing clinician or pharmacist before changing how you take metformin.

References

  1. 1.American Diabetes Association Professional Practice Committee (2024). Standards of Care in Diabetes—2024. Diabetes Care. doi:10.2337/dc24-SINT ✓Recommendations for metformin as first-line therapy, gradual dose titration for GI tolerability, extended-release formulation for GI side effects, periodic B12 monitoring, and kidney function thresholds for dose adjustment.
  2. 2.MedlinePlus / U.S. National Library of Medicine (2024). Metformin: MedlinePlus Drug Information. MedlinePlus / NLM. link ✓Lactic acidosis risk factors, contrast dye interaction, kidney function requirements, and standard prescribing information referenced in the article.
  3. 3.Obeid R, Andrès E, Češka R, et al. (2024). Diagnosis, Treatment and Long-Term Management of Vitamin B12 Deficiency in Adults: A Delphi Expert Consensus. Journal of Clinical Medicine. doi:10.3390/jcm13082176 ✓B12 deficiency mechanism with metformin use, symptom progression (fatigue, neuropathy), and recommendation for monitoring and supplementation in long-term users.

3 sources, numbered by first appearance. General health information, not medical advice — synthetic demonstration content.