In short
- MedlinePlus lists a set of side effects for metformin, most of them affecting the stomach and gut.
- It says to tell your doctor if a side effect is severe, does not go away, keeps coming back, or starts some time after you began the medicine.
- A short list of symptoms needs a call to a doctor straight away, and some need emergency services. Those are set out below.
- Nothing on this page tells you to change how you take your medicine. That conversation belongs with your prescriber.
Side effects MedlinePlus lists
MedlinePlus, the US National Library of Medicine's consumer health service, publishes the following side effects for metformin. They are grouped here to make them easier to read; the grouping is this site's, not the source's.
| Group | Side effects named |
|---|---|
| Stomach and gut | Diarrhea, nausea, stomach discomfort, gas, indigestion, constipation |
| Energy and general | Lack of energy or weakness, headache, muscle pain |
| Taste, skin and nails | A change in the sense of taste, flushing of the skin, rash, nail changes |
NIDDK's general advice on diabetes medicines points the same way. It says to ask your doctor whether your diabetes medicine can cause side effects such as an upset stomach, and to take your medicines as your doctor instructs, which it says helps prevent side effects and diabetes problems.
MedlinePlus also says metformin may cause other side effects that are not on its list, and to call your doctor about any unusual problem while taking it.
When to mention a side effect
MedlinePlus gives four situations in which to tell your doctor about any of the side effects above:
- it is severe;
- it does not go away;
- it goes away and then comes back;
- it does not begin until some time after you started the medicine.
The last point is easy to miss. A side effect that appears months in is still worth mentioning, even if the first weeks went smoothly.
FDA's general guide to the benefits and risks of medicines suggests asking your clinician or pharmacist whether there is anything you can do to reduce side effects. For any medicine, that is the right question to bring to them, rather than changing how you take it on your own. MedlinePlus is explicit on that point for diabetes medicines: do not change or stop them without talking to your provider first.
Changes in blood glucose
MedlinePlus says metformin may cause changes in blood glucose, and that people taking it should know the symptoms of both low and high blood glucose and what to do if they happen. "What to do" is part of each person's plan, so it is worth asking your care team to go through it with you.
Low blood glucose (hypoglycemia). NIDDK and MedlinePlus say lows are most common with insulin and with some other medicines that make the pancreas release more insulin. NIDDK names two such groups of pills, sulfonylureas and meglitinides. NIDDK also lists triggers that can lower glucose in anyone taking diabetes medicines: not eating enough carbohydrate or skipping or delaying meals, more physical activity than usual, alcohol without food, and being ill. MedlinePlus's metformin page adds that alcohol can lower blood glucose.
Symptoms of a low, as listed by NIDDK and MedlinePlus, include feeling shaky, sweating, nervousness or anxiety, irritability or confusion, dizziness, hunger, a fast or uneven heartbeat, headache, and trouble seeing or speaking clearly. A very low level can lead to passing out or a seizure. NIDDK notes that lows can also happen during sleep, with signs such as nightmares, sweating enough to dampen bedding, or waking tired and confused. CDC describes hypoglycemia unawareness, where someone does not feel the usual warning signs, which makes lows harder to catch.
NIDDK publishes a step-by-step method for treating a mild low, and MedlinePlus explains that a doctor can prescribe glucagon for severe lows. Which of these applies to you depends on your medicines, so ask your care team for your own plan rather than relying on a general page.
High blood glucose (hyperglycemia). MedlinePlus and NIDDK list feeling thirsty, feeling tired or weak, headaches, urinating often and blurred vision. MedlinePlus says that if you often have high readings or these symptoms, talk with your care team, because your meal plan, activity or medicines may need to change.
Vitamin B12
MedlinePlus's metformin page asks people to tell their doctor if they have ever had low levels of vitamin B12. The sources used for this site do not go further than that, so this page does not either. If you want to know whether vitamin B12 is relevant to you, it is a reasonable question for your clinician, who can decide whether any test is needed.
Symptoms that need a call now
Emergency
MedlinePlus says to call emergency services (911 in the US, or your local emergency number) if someone has collapsed, had a seizure, has trouble breathing or cannot be woken. It also lists chest pain as a reason to call your doctor immediately or get emergency treatment.
MedlinePlus's important warning on metformin is about a rare but serious condition called lactic acidosis, covered in the guide on serious risks. It lists these symptoms as needing an immediate call to your doctor:
- extreme tiredness, weakness or discomfort
- nausea or vomiting
- stomach pain
- decreased appetite
- deep and rapid breathing, or shortness of breath
- dizziness or lightheadedness
- a fast or slow heartbeat
- muscle pain
- feeling cold, especially in the hands or feet
Several of these overlap with the everyday side effects above, which is why the source treats them as a set. The same warning section also says what to do about the medicine itself in this situation. Read that section with your prescriber or pharmacist when you first start, so you already know your plan if it is ever needed.
For a severe low, NIDDK says family, friends or coworkers who give glucagon should call 911 straight away afterwards, or call 911 if no glucagon kit is at hand.
Keeping notes
FDA suggests paying attention to how you feel on any medicine and writing down changes so you remember to tell your doctor, pharmacist or other clinician. A short note with the date, what you noticed, how long it lasted and anything that seemed to set it off is much easier to discuss than a memory.
Worked example
This example is invented and describes no real person. A note reads: "Week 3. Loose stools most mornings, about four days running. Better at weekends. No fever, eating normally." It records what MedlinePlus asks about (how severe, whether it goes away or comes back) without guessing at a cause, and it gives the clinician something concrete to work with.
Reporting a problem to FDA
MedlinePlus says that if you have a serious side effect, you or your doctor can report it to FDA's MedWatch program. FDA describes MedWatch as its safety reporting program for health professionals, patients and consumers, covering prescription and over-the-counter medicines among other products. FDA says MedWatch receives reports from the public and, when appropriate, publishes safety alerts. A report does not replace getting medical care for the problem itself.
Next: Rare but serious risks: lactic acidosis, kidney function and dye scans