Metformin's side effects are well mapped on its label. It lists the most common ones as "diarrhea, nausea/vomiting, flatulence, asthenia, indigestion, abdominal discomfort, and headache" [1]. It is approved for type 2 diabetes; people taking it for longevity or weight are using it off-label and face the same list. Here is how often each side effect happened, and which ones matter more over years.
Stomach effects top the list, led by diarrhea
The label reports two sets of trial numbers: one trial of immediate-release tablets, the original form, and a pool of trials of extended-release tablets, which release the drug slowly. They were separate trials, so the two columns aren't a head-to-head comparison [1].
| Side effect | Immediate-release | Placebo | Extended-release | Placebo |
|---|---|---|---|---|
| Diarrhea | 53% | 12% | 10% | 3% |
| Nausea or vomiting | 26% | 8% | 7% | 2% |
| Gas (flatulence) | 12% | 6% | 1% to 5% | Less often |
| Weakness (asthenia) | 9% | 6% | Not listed | Not listed |
| Indigestion | 7% | 4% | 1% to 5% | Less often |
| Abdominal discomfort | 6% | 5% | 1% to 5% (abdominal pain) | Less often |
| Headache | 6% | 5% | 1% to 5% | Less often |
| Stopped because of diarrhea | 6% | Not given | 0.6% | Not given |
Immediate-release: 141 people on metformin, 145 on placebo. Extended-release: 781 on metformin, 195 on placebo. Metformin prescribing information, Tables 1 and 2 and section 6.1 [1].
In plain terms, about half the people on immediate-release metformin in that trial had diarrhea at some point, against about 1 in 8 on placebo. It was common enough that 6% stopped the drug over it [1]. Weakness is covered in more depth in does metformin make you tired?
The label doesn't explain why diarrhea is so common
You'll find theories elsewhere about how metformin acts in the gut. The label offers none; it describes how metformin lowers blood sugar and stops there [1]. What it does show is the gap between forms: the extended-release trials reported much lower rates of diarrhea and nausea than the immediate-release trial. Because those were different trials with different people, the label doesn't claim one form is gentler, and neither should we. Which form suits you is your prescriber's call.
Stomach side effects usually ease with time
The patient Medication Guide says: "Common side effects … include diarrhea, nausea, and upset stomach. These side effects generally go away after you take the medicine for a while" [1]. The guide adds that taking it with meals can help, and that is the label's own instruction; your prescriber sets how you take it.
Long-term data agree. In the Diabetes Prevention Program's follow-up, which tracked people for about 10 years, stomach symptoms were more common on metformin than placebo "and declined over time" [2]. The guide also says when to call: tell your doctor if side effects "bother you a lot, last for more than a few weeks, come back after they've gone away, or start later in therapy" [1].
Vitamin B12 can drop over years of use
This is the side effect that matters most for people who plan to stay on metformin for the long run. In 29-week trials, about 7% of people with normal vitamin B12 developed low levels. The label says the drop "may be associated with anemia but appears to be rapidly reversible" when metformin is stopped or B12 is supplemented, and it sets a monitoring rule: "Measure hematologic parameters on an annual basis and vitamin B12 at 2 to 3 year intervals" [1].
Over a longer stretch the share grows. In the Diabetes Prevention Program's follow-up, low or borderline-low B12 was found in 19.1% on metformin vs 9.5% on placebo at 5 years, and 20.3% vs 15.6% at 13 years. Each year of use raised the odds of deficiency by about 13% [3]. If you take metformin for longevity, as covered in metformin for longevity, ask whether your B12 test is due.
The serious one: lactic acidosis, rare and in the boxed warning
Lactic acidosis is a buildup of lactic acid in the blood. On metformin it is rare, but the label's boxed warning says reported cases "have resulted in death" [1]. Its early signs are vague: the label lists "malaise, myalgias, respiratory distress, somnolence, and abdominal pain," which in everyday words means feeling unwell, muscle pain, trouble breathing, unusual sleepiness and stomach pain.
| Risk factor | What the label says |
|---|---|
| Kidney problems | Not to be used when kidney function (eGFR) is below 30; kidney function checked at least once a year |
| Age 65 or older | Kidney function checked more often |
| Contrast imaging tests | Stopped at or before the test for some people; tell the team you take metformin |
| Surgery and other procedures | Paused while food and fluids are restricted |
| Excessive alcohol intake | Patients are warned against it |
| Liver disease | Metformin is avoided |
| Low-oxygen states such as acute heart failure, heart attack or sepsis | Metformin is stopped if one occurs |
| Some other drugs, such as topiramate | Closer monitoring |
| Mitochondrial diseases such as MELAS | Not recommended |
Metformin prescribing information, Boxed Warning, sections 4 and 5.1, and Medication Guide [1].
The guide also lists dehydration, from fever, vomiting, diarrhea or heavy sweating, as something that raises the risk [1]. Drinking is covered in detail in alcohol on a GLP-1 or metformin.
Low blood sugar is rare on metformin alone
The Medication Guide says metformin tablets "rarely cause hypoglycemia (low blood sugar) by themselves," but that it can happen if you don't eat enough, drink alcohol or take other blood-sugar medicines [1]. The label adds that combining metformin with insulin or a sulfonylurea may raise the risk, and that those doses may need lowering [1].
Constipation is listed; dry mouth isn't
Constipation is on the label, in the extended-release list of side effects reported by 1% to 5% of people, alongside abdominal pain, bloating, heartburn, gas, dizziness, headache, upper respiratory infection and a change in taste [1]. The immediate-release list at the same 1% to 5% level includes abnormal stools, sweating, a taste disorder, muscle pain and lightheadedness.
Dry mouth doesn't appear anywhere on the current label, and neither does hair loss [1]. After approval, liver injury has also been reported with metformin, without a known rate [1].
If side effects are getting in the way
If stomach side effects bother you, the Medication Guide notes that you may need a lower dose or a break from the medicine, which only your prescriber can decide [1]. If you're comparing options, metformin vs Ozempic sets the two side by side, metformin and weight loss covers what it does for weight, and getting metformin prescribed online explains the visit and labs. Programs that prescribe it, including AgelessRx, HealthRx and Found, are scored in metformin programs online.