Metformin: what it treats, how to take it, side effects
Last updated September 3, 2026.
Metformin is a biguanide medication used, alone or with other drugs, to control blood sugar in type 2 diabetes. It works by decreasing the glucose you absorb from food and the glucose your liver makes, and by improving your body's response to insulin. It is not used for type 1 diabetes.
What does it treat?
Metformin treats type 2 diabetes, the condition where the body does not use insulin normally and blood sugar runs high. Controlling blood sugar matters because over time high sugar damages the heart, kidneys, nerves, and eyes. MedlinePlus frames medication, diet, exercise, quitting smoking, and regular blood sugar checks as one package - metformin is the medication part, not a substitute for the rest.
How do you take it?
Always with meals, which softens the stomach side effects. The regular tablet is usually taken two or three times a day with meals, the liquid once or twice a day with meals, and the extended-release tablet once a day with the evening meal - swallowed whole, never split or crushed. Your doctor will start you on a low dose and increase it no more often than every 1 to 2 weeks, watching your blood sugar results. Metformin controls diabetes but does not cure it, so keep taking it even when you feel well. If you miss a dose, take it when you remember unless the next one is close; never take a double dose.
Side effects to know
- Common: diarrhea, nausea, stomach discomfort, gas, indigestion, constipation, lack of energy, a change in taste, and headache - often worst when starting or increasing the dose.
- Serious: lactic acidosis, a rare but life-threatening buildup of acid in the blood. Its symptoms include extreme tiredness or weakness, nausea, vomiting, stomach pain, decreased appetite, deep and rapid breathing or shortness of breath, dizziness, a fast or slow heartbeat, muscle pain, and feeling unusually cold, especially in the hands and feet. Stop taking metformin and get emergency care if these appear. Chest pain also requires immediate attention.
Who should not take it?
Kidney function decides a lot: the label says do not use metformin when eGFR is below 30 mL/min/1.73 m2, and starting is not recommended between 30 and 45 - your doctor will check your kidneys before and during treatment. The risk of lactic acidosis also rises if you are over 65, have liver disease or heart failure, drink alcohol heavily, or become severely dehydrated or seriously ill. Planning surgery or a scan with injected contrast dye? You will likely need to stop metformin beforehand and wait at least 48 hours before restarting - your doctor sets the exact timing. Tell your doctor if you are pregnant, planning pregnancy, or breastfeeding, and about all other medications, especially topiramate and similar drugs.
When is it an emergency?
Lactic acidosis is the emergency: extreme tiredness, trouble breathing, stomach pain with vomiting, dizziness, slow or irregular heartbeat, muscle pain, and feeling cold mean stopping the medication and calling 911 or getting to an emergency room. For an overdose, call Poison Control at 1-800-222-1222 - hypoglycemia symptoms can appear with an overdose. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why do I have to take metformin with food?
MedlinePlus's dosing instructions are built around meals - with meals for the regular tablet and liquid, with the evening meal for extended-release. Food blunts the stomach side effects (nausea, diarrhea, gas) that make people quit metformin early.
Can I drink alcohol while taking metformin?
This one deserves a real conversation with your doctor. MedlinePlus warns that drinking alcohol - regularly, or binge drinking occasionally - increases the risk of lactic acidosis and can also drop blood sugar. Ask your doctor how much, if any, is safe for you.
Why did I have to stop metformin before my CT scan?
The FDA label directs stopping metformin at the time of, or before, an iodinated contrast imaging procedure for patients with reduced kidney function, a history of liver disease, alcoholism, or heart failure, or with intra-arterial contrast. Kidney function is rechecked 48 hours later and metformin restarts only if it is stable. The contrast can stress the kidneys, and metformin plus struggling kidneys raises lactic acidosis risk.
Does metformin cause low blood sugar?
MedlinePlus emphasizes knowing the symptoms of both low and high blood sugar while on metformin, and lists hypoglycemia symptoms among possible overdose effects. Skipping meals, unusual exercise, or combining metformin with other diabetes medications can push sugar too low - your doctor will tell you what to watch for.
How long does metformin take to lower blood sugar?
It works gradually, which is built into the dosing: MedlinePlus says doctors start low and increase no more often than every 1 to 2 weeks while you monitor your blood sugar. Expect weeks of titration, not an overnight change.
Why does metformin cause diarrhea, and does it go away?
Diarrhea, nausea, and stomach discomfort top MedlinePlus's list of common metformin side effects, and they cluster around starting the drug and each dose increase. For many people they fade as the body adjusts - if yours are severe, keep coming back, or start long after you began the drug, tell your doctor rather than pushing through.