If you have type 2 diabetes, there's a good chance your doctor will recommend you start taking the drug metformin, which helps control the amount of glucose, or sugar, you have in your body.
"It decreases the amount of sugar you absorb from your food, and it also lowers the amount of sugar made by your liver," says Nirali Shah, MBBS, an endocrinologist with the Mount Sinai Health System in New York City. "It also increases your body's response to insulin, a hormone that controls the amount of glucose in your blood."
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This tried-and-true medication has been around for over a quarter century — it was approved by the FDA for type 2 diabetes in 1995. "It's very effective and very safe, which is why it's still the first-line drug used to treat type 2 diabetes," Dr. Shah says.
But like any drug, metformin has side effects, and there are some dos and don'ts when it comes to taking it.
Here are nine things experts want you to know when you're starting metformin:
1. You Could Have GI Issues, Including Diarrhea
"The main side effect people notice is an upset stomach, especially diarrhea, because metformin is irritating to the stomach lining," says Evan Sisson, PharmD, a professor at the VCU School of Pharmacy in Richmond, Virginia.
So, how long does diarrhea last when starting metformin? It usually gets better within the first week to 10 days, Sisson says. The best way to decrease this side effect is to have some food already in your stomach when you take it.
"It provides a coating, so the metformin doesn't run right next to the lining and cause discomfort," Sisson explains.
He recommends you start your meal, then take your metformin halfway through.
When you are first starting metformin, it also helps to build up your dose slowly, Dr. Shah says.
"I usually start patients on a 500-milligram dose, which is the lowest dose available, then gradually increase it to 750 or 1,000 milligrams," she says.
If you still have issues, you can try metformin extended-release. "It releases the drug slowly, so you get small amounts of it through the entire day," Dr. Shah explains. "People are often able to tolerate that better than taking all of the drug at once."
2. It Can Regulate Your Periods
You may notice you have what seems to be a heavy period after starting metformin. It isn't.
"Some women with diabetes also have a condition called polycystic ovary syndrome (PCOS), a condition where your body has trouble with many hormones, including insulin and the male hormone androgen," Dr. Shah says.
PCOS causes fluid-filled cysts on your ovaries and also raises your risk for developing type 2 diabetes — in fact, more than half of people with PCOS go on to be diagnosed with type 2 diabetes after age 40, according to the Centers for Disease Control and Prevention.
"If you have PCOS, you have very irregular cycles, where you may go months without getting a period," Dr. Shah says. "Once you start metformin, those cycles become regulated. It might seem like your period is heavier, when really you're just not used to having one at all."
3. You'll Need to Watch Your Alcohol Intake
It's not that alcohol itself actually interacts with metformin, Dr. Shah says. "Alcohol can suppress your body's production of sugar if you have more than two drinks," she explains. "Metformin does that too, so the combination of both can lead to low blood sugar levels."
If you're having just a glass or two of wine at dinner, it's OK, Dr. Shah says. But don't have more than that, and when you do imbibe, try to pair it with food.
"Snacks with protein or fat in them can keep your blood sugar up for four to six hours," she adds. "That's one of the reasons why bars serve nuts."
4. You’ll Need to Be Screened for Anemia
Metformin can decrease the amount of vitamin B12 you absorb, which can lead to vitamin B12 deficiency and vitamin-deficiency anemia, says Thomas So, PharmD, Senior Manager of Consumer Drug Information Group at First Databank, Inc, in San Francisco, California.
A November 2019 study in Medicine that included more than 1,000 people with type 2 diabetes taking metformin found that about 20 percent had low vitamin B12 levels. The risk seems to increase the higher your dose (but it's not affected by how long you've taken the drug).
Your doctor can run blood tests to check your red blood cell count and vitamin B12 levels. If you're diagnosed with vitamin-deficiency anemia, treatment may include diet changes and vitamin B12 supplements or injections.
5. Metformin May Cause You to Lose Weight
According to a June 2019 review in Current Obesity Reports, some research has linked metformin with weight loss.
"The medication seems to cause a slight decrease in appetite," notes Gary Scheiner, a certified diabetes educator and owner of Integrated Diabetes Services in Wynnewood, Pennsylvania.
This may be because it makes your body more sensitive to leptin — a hormone that tells your body when you're full, per the Cleveland Clinic.
Other research, per the Current Obesity Reports review, suggests metformin may increase secretion of the GLP-1 hormone, which lowers your appetite as well.
And there's one more reason why metformin may also help you shed pounds: "If your blood sugar is well controlled, you're less likely to be excessively hungry," Dr. Shah says.
6. You Could Experience Some Changes in Vision
You may notice blurred vision after starting metformin. If you do, let your doctor know.
"It could be a sign that your blood sugar isn't well controlled," says Sisson. "Sugar draws water in, so when your sugar levels are high, it can temporarily increase fluid in the eye and change the focal length between your lens and retina. That's why some people with diabetes notice their vision is blurry, and they can't read the newspaper after they eat dinner."
Your doctor can work with you to monitor your blood glucose levels, to make sure they stay in the right range. If they are, this may just be a temporary side effect that goes away on its own.
The good news is that metformin itself may actually be protective against vision loss: A September 2018 study in the Proceedings of the National Academy of Sciences found that the drug stimulates enzymes that help protect retinal cells and makes you less likely to develop age-related macular degeneration.
7. Metformin Has Been Linked to a Rare but Serious Side Effect
Some research has linked metformin to a condition called lactic acidosis, Dr. Shah says. This is when your body produces too much of a substance called lactic acid before it can get rid of it, and it can lead to life-threatening complications. Thankfully, it's very rare.
You may be at higher risk for lactic acidosis if you have severe kidney, liver or heart disease, per the Mayo Clinic.
Even if you don't have these conditions, you should know the warning signs of lactic acidosis: GI problems such as stomach pain or diarrhea that are accompanied by other symptoms like fast, shallow breathing, muscle cramps or feeling unusually sleepy, weak or tired. If you develop these symptoms, you should call your doctor immediately, Dr. Shah says.
8. The Pills Are Large
This may not sound like a big deal, but if "you're someone who has difficulty taking pills, you may find it challenging," says Scheiner.
You can try the lean-forward method:
- Put the pill on your tongue
- Take a sip of water (but don't swallow it)
- Tilt your chin toward your chest
- Now, swallow the pill and water while your head is bent
This led to an 89 percent improvement over just taking a sip of water from a cup among people who reported trouble swallowing pills, according to a November 2014 report in the Annals of Family Medicine.
But if you still have problems, don't worry: There's also a liquid form of metformin you can take, Scheiner says.
9. You Can’t Rely on Metformin Alone
Metformin is a very effective drug to treat type 2 diabetes. Because it works so well, and is very safe, it's often the first-line treatment most doctors will prescribe, says Dr. Shah. But you can't just use it in isolation.
"To get the best results from this drug, it's very important to follow a healthy lifestyle, which includes eating right and exercising," Dr. Shah stresses. This way, you'll be best able to manage your type 2 diabetes.
- US Food & Drug Administration: "FDA Drug Safety Communication: FDA revises warnings regarding use of the diabetes medicine metformin in certain patients with reduced kidney function"
- Current Obesity Reports: "Metformin: Mechanisms in Human Obesity and Weight Loss"
- Proceedings of the National Academy of Sciences: "Stimulation of AMPK prevents degeneration of photoreceptors and the retinal pigment epithelium"
- Annals of Family Medicine: "Two Techniques to Make Swallowing Pills Easier"
- Centers for Disease Control and Prevention: "PCOS (Polycystic Ovary Syndrome) and Diabetes"
- Medicine: "Association between metformin dose and vitamin B12 deficiency in patients with type 2 diabetes"
- Cleveland Clinic: "Leptin & Leptin Resistance"
- Mayo Clinic: "Glyburide And Metformin (Oral Route) Precautions"
Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.