Metformin is one of the first treatment modalities developed to treat type 2 diabetes, alongside insulin and sulfonylureas. After metformin was discovered and developed in the 1950s-60s, it quickly became a main treatment option to help lower blood sugars. To this day, it remains the initial option for the treatment of type 2 diabetes. However, many individuals struggle to tolerate the immediate-release (IR) formulation of metformin, which prevents them from benefiting from a low-cost, effective therapy for type 2 diabetes.
Fortunately for patients who are unable to tolerate the immediate-release formulation, metformin is also available as a sustained-release formulation, providing a steady concentration of the drug over a 24-hour period. The goal of this article is to explain how metformin benefits diabetes and explore whether sustained-release metformin is better tolerated compared to immediate-release metformin, along with other diabetes treatment options.
What is Metformin Sustained Release?
Metformin is most commonly used for type 2 diabetes. However, it is also used off-label for prediabetes, polycystic ovary syndrome (PCOS), antipsychotic-induced weight gain, and gestational diabetes. In PCOS, metformin may help regulate menstrual cycles, reduce androgen levels, and improve fertility. In prediabetes, it may delay or prevent progression to full diabetes in individuals with high risk. Because of its broad impact on insulin sensitivity and weight regulation, it has also gained traction in off-label uses where metabolic disturbances are involved.
Metformin is available in several formulations, including immediate-release (IR), sustained-release (SR), and a liquid solution. You may see a few different terms used interchangeably with sustained-release, including extended-release (ER) or XR (also meaning extended release). When it comes to metformin formulations, all of these refer to slow-release formulations that can be taken once daily.
The following table shows the different formulations of metformin and how they differ:
| Parameter | Metformin IR | Metformin SR (ER/XR) |
| Brand Examples | Glucophage | Glucophage XR, Fortamet, Glumetza |
| Dosing Frequency | 2-3 times daily with meals | Once daily (typically with evening meal) |
| Absorption Site | Small intestine (duodenum) | Distal small intestine and colon |
| Peak Plasma Time | ~ 3 hours | ~ 6 to 8 hours |
| GI Tolerability | Lower (more nausea, diarrhea) | Improved tolerability |
| Cost/Availability | Widely available, low cost | Available, slightly higher cost |
| Adherence Potential | Lower due to multiple daily doses | Higher with once-daily dosing |
| Tablet Appearance | Small, often scored | Larger; Glumetza and Fortamet are non-scored |
| Clinical Considerations | Titrate slowly to minimize GI side effects | Do not crush/chew; not interchangeable across brands |
How Does Metformin Work in the Body?
Understanding Biguanides and Blood Sugar Regulation
Metformin belongs to a drug class called biguanides and is considered an antidiabetic medication. The primary mechanism of action of metformin is to reduce glucose production in the liver by inhibiting a process called gluconeogenesis. It also helps muscle and liver tissues utilize insulin better, improving the impact insulin has on lowering blood sugar (often referred to as insulin sensitivity). Metformin is thought to have additional mechanisms that are not completely understood.
Compared to insulin and sulfonylureas (glipizide, glimepiride, etc.), metformin has a low risk of causing hypoglycemia, also known as low blood sugar. Low blood sugar is a blood glucose reading lower than 70 mg/dl. Patients with a history of significantly uncontrolled diabetes may experience hypoglycemia symptoms at a level higher than 70 mg/dl. These symptoms include dizziness, shakiness, and sweating.
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Let’s take Jim as an example. Jim was recently diagnosed with type 2 diabetes. He was nervous about starting metformin due to the stomach side effects, so his doctor agreed to prescribe the sustained-release formulation. He may benefit from the following:
- Fewer stomach issues: no racing to the restroom during working hours.
- Once-daily dosing: it fits right into his morning routine, which includes 1-4 tablets with breakfast, depending on his dose.
- Slow-Release: no sharp blood sugar dips or peaks, improving the way he felt throughout the day
- Similar efficacy: it works just as well as the IR version for lowering blood sugar.
How to Take Metformin Sustained-Release the Right Way
There are a few strategies when taking sustained-release metformin that can reduce side effects and maximize the effects of the medication:
- Take it with your evening meal, or the largest meal of the day.
- Taking it in the evening may help manage overnight and post-dinner blood sugars.
- Taking it with food can help reduce nausea and cramping, even with the sustained-release metformin.
- Maintain consistent timing each day and set reminders as needed.
- If you are prone to stomach side effects like nausea or diarrhea, avoid taking sustained-release metformin on an empty stomach
Missed Dose of Metformin: What Should You Do?
Even if you are consistent and able to implement the ideal strategies described, it is possible to miss doses of your medication accidentally. If you do miss a dose of sustained-release metformin, do not double up on medication, as it will increase the chance of you experiencing stomach issues. If it is approaching the time of your next dose, just skip the missed dose and resume your normal schedule. If you are unsure whether to take a missed dose, consult your pharmacist or doctor for guidance.
Side Effects of Metformin Sustained Release
As with other formulations of metformin, the use of sustained-release tablets can also lead to the following side effects:
- Vitamin B12 deficiency
- Abdominal distress or pain
- Headache
- Dyspepsia (indigestion)
- Fatigue
Although the sustained-release may also cause these side effects, the incidence of a few side effects differs compared to the immediate-release (IR) formulation.
| Side Effect | Metformin IR | Metformin SR |
| Diarrhea | 53% | 10–20% |
| Nausea | 26% | 7–10% |
| Abdominal discomfort | 12% | 6–9% |
| Flatulence | 12% | 5–7% |
| Loss of appetite | 5–10% | 2–5% |
| Metallic taste | 3–5% | <2% |
| Vitamin B12 deficiency* | ~6% (long-term) | ~6% (long-term) |
| Lactic acidosis | Rare (<0.1%) | Rare (<0.1%) |
Note: Vitamin B12 deficiency risk increases with use over >12 months, regardless of formulation.
What Are Metformin Ghost-Tablets?
One unique side effect of slow-release formulations is the potential for a metformin shell to be visible in the stool. Sustained-release metformin contains an outer shell that is made up of a non-digestible polymer (case) that may present whole or partially in the stool after the active medication is released gradually in the gastrointestinal tract.
If you see this metformin ghost shell, do not panic. This side effect is harmless and expected. The drug has already been absorbed, and the ghost tablet is just the leftover structure.
Does Metformin Cause Diarrhea?
Metformin commonly causes diarrhea, particularly in the first few weeks of starting the medication or after a dose increase. Over time, it is likely that your body will adjust and get used to the medication, leading to fewer side effects. If you are experiencing excessive diarrhea with immediate-release metformin a few weeks after being on a consistent dose, you and your doctor may decide to switch over to the sustained-release formulation, which has a lower incidence of diarrhea.
Metformin Doses and Titration
Metformin hydrochloride is available in two oral tablet formulations: immediate-release (IR) and extended-release (ER). Each requires a slightly different dosing strategy based on how the medication is absorbed throughout the day. The IR version reaches peak concentration in about 2 to 3 hours, while the ER formulation is absorbed more slowly, peaking at around 6 to 8 hours.
For both versions, the typical starting dose is 500 mg once daily, usually taken with food to help reduce gastrointestinal side effects such as nausea, bloating, or diarrhea.
Doses are generally increased by 500 mg each week, depending on tolerance, until the maximum effective or tolerated dose is reached. Monitoring for side effects is essential during this titration period, as some patients may need to stay at a given dose longer to allow their body to adjust. This gradual titration approach helps minimize uncomfortable side effects and improve adherence.
In older adults (?80 years) or those with renal impairment (e.g., eGFR between 30–45 mL/min), the starting dose remains 500 mg once daily. However, the maintenance dose is often limited to 500 mg twice daily, with frequent monitoring of kidney function to ensure continued safety. To help your body adjust, it’s recommended to stay on each dose for at least one full week before increasing to the next level.
What If You Need to Stop Metformin?
There are times when discontinuing metformin may be necessary. This can include intolerable side effects, a change in kidney function, or transitioning to insulin or another diabetes medication due to disease progression. If you need to stop metformin, it should always be done under the guidance of your healthcare provider. Abruptly stopping without a plan could result in poor blood sugar control. Your provider may either taper the dose or stop it and replace it with another agent, depending on your situation.
Stopping Metformin: When and How
Common reasons for stopping metformin include worsening kidney function (e.g., eGFR <30 mL/min), ongoing gastrointestinal side effects despite switching to the extended-release version, or switching to alternative therapies like SGLT2 inhibitors or GLP-1 receptor agonists. Patients undergoing contrast imaging procedures are often advised to hold metformin temporarily to reduce the risk of lactic acidosis. In these cases, metformin can usually be resumed after 48 hours once kidney function is reassessed and stable.
Can You Crush Metformin Sustained-Release Tablets?
Although crushing pills can improve the ease of administration, some medications should not be crushed. The following are examples of what could happen if pills are crushed that shouldn’t be:
- Crushing extended-release medications can result in getting a large dose all at once.
- Crushing delayed-release medication can alter the mechanism designed to protect the drug from acidic environments.
- Crushing sublingual (placed under the tongue) tablets can alter the effectiveness of the medication.
- Crushing some medications may result in a bitter taste when swallowed.
- Crushing certain medications can lead to mouth irritation.
- Crushing certain medications may result in less of the medication getting absorbed in the body.
The Pharmacist’s Letter, an online resource for pharmacists and other healthcare professionals, issues and updates a resource that lists medications that should not be crushed. According to this list, the slow-release formulations of metformin should not be crushed. This includes any sustained-release (SR) or extended-release (XR/ER) formulations. Examples of these include Fortamet and Glumetza.
Conclusion: Is Metformin Sustained Release Right for You?
If you’ve experienced stomach upset, diarrhea, or other intolerances with immediate-release metformin, the sustained-release formulation may offer a smoother experience. With once-daily dosing and fewer gastrointestinal side effects, it can help patients stay consistent with treatment while achieving the same blood sugar-lowering effects. Always speak with your healthcare provider about which formulation is best for your health needs, lifestyle, and tolerance. In many cases, sustained-release metformin can be the key to staying on therapy without sacrificing comfort.
