How Long Does Metformin Stay in Your System?

Understanding how long metformin stays in your system can help optimize its benefits and manage potential side effects.

It’s also relevant if you are stopping metformin and want to know when the drug has completely left your system.

Why is it important to know how long metformin stays in your system?

Metformin is generally best taken once or twice daily. (Be sure to check with your doctor about the best schedule for your situation.) 

But without knowing how long it stays in your system, it can be difficult to know how and when to take your medicine if you’re trying to raise or lower your dose or to wean yourself off metformin altogether. 

It is additionally helpful to know how long metformin lasts in your system if you’ve forgotten to take your medication, if you are wondering when it is safe to take your next full dose, or if you’re not sure whether the metformin you’ve previously taken will interact with other medications or food or alcohol you plan to consume.

Does metformin take a while to build up in your system?

Yes, and if you’ve just started taking metformin, you most likely won’t notice improved blood sugar levels or weight loss immediately, no matter what dose you are taking. 

The first effects on blood sugar levels may be noticed within 48 hours of starting the medication, but the most significant effects won’t be seen until after 4-5 days of consistently taking the medication. 

Any expectations of weight loss, however, may take several weeks or months to come to fruition and will require changes in diet and exercise as well. It’s also important to note that the weight loss generally associated with metformin is modest — in the range of 5 pounds. 

Read more in: Signs Metformin Is Working (Or Isn’t Working)

A potentially dangerous buildup of metformin in the body can happen if your kidneys are not properly functioning to process the medication. This can result in a condition called lactic acidosis (a buildup of lactic acid in the bloodstream), which can be life-threatening. 

Contact your doctor if you’re experiencing symptoms of lactic acidosis, including:

• Feeling disoriented
• Rapid breathing
• Rapid heart rate 
• Abdominal pain
• Headache
• Lethargy and fatigue
• Muscle cramps and muscle pain
• Body weakness
• Reduced appetite
• Diarrhea
• Vomiting
• Nausea
• Yellowing of the skin or eyes

Some of these symptoms can be confused with normal metformin side effects, but take note if you start to experience any new symptoms after you’ve been on metformin for a while. 

(All this being said, metformin is generally a very safe drug, and most people experience minimal side effects.)

How long do metformin side effects last?

Typically, side effects of metformin, such as vomiting, diarrhea, nausea, stomachache, and loss of appetite, go away after a few weeks on the medication. 

For those who experience significant side effects, there are good alternatives to metformin, including several metformin combination drugs that may have fewer side effects.

Metformin half-life

The half-life of a medication is the time it takes for 50 percent of the drug’s active ingredient to be metabolized or eliminated from your body. 

Metformin has an elimination half-life from red blood cells of approximately 17.6 hours. (It is important to note, however, that this figure may vary between different formulations of the medication, such as extended-release versus immediate-release.)

The average elimination half-life of metformin in plasma (the liquid portion of blood) is only 6.2 hours, but most of the active ingredients in metformin accumulate in the red blood cells (RBCs), so that is where we will focus. 

This means that every 17.6 hours, the drug becomes 50 percent less strong. However, there are some caveats.

Does metformin stay in some people’s systems longer than others? 

Yes. The half-life of metformin is not an exact science because everyone is different. Metformin tends to remain in the bodies of certain populations for a longer time.

People with renal (kidney) impairment

For people living with kidney disease or kidney failure, metformin will stay in the body much longer. 

For people taking metformin who have mild kidney failure, the oral clearance (the rate at which the body processes and absorbs metformin from the gastrointestinal tract into the bloodstream) and renal clearance (the rate at which the kidneys eliminate metformin from the body) of metformin is decreased by 33 percent and 16 percent, respectively. 

For those with moderate kidney failure, the oral clearance and renal clearance of metformin are decreased by 50 percent and 53 percent, respectively. 

Age

Older people tend to have metformin stay in their system longer than younger people. This has a few causes, but the major one is decreased kidney function as people with diabetes age. 

For that reason, it is recommended that metformin should not be started in people who are 80 years and older unless creatinine clearance (a measure of how well the kidneys filter creatinine out of the bloodstream) can show that kidney function is not reduced.

Metabolic rate 

People who have a slower metabolic rate will have metformin in their system for longer. This is because the body is processing everything (including food, alcohol, and medications) more slowly. 

Compared to people without diabetes, those with type 2 diabetes who take metformin will have the medication in their system for longer, even after stopping the drug. 

Body mass

The higher your body mass, the longer metformin will stay in your system. This applies to all medications. 

If your body mass is higher, you’re also more likely to be taking a larger dose of metformin, which will affect the time it spends in your system as well. 

Dose 

If you’re on a low dose of metformin, the duration it stays in your system is lower than for someone who is on the maximum daily dose of metformin, which is roughly 2,550 milligrams (mg) per day. 

If you’re taking the maximum dose, expect the medication to stay in your system much longer than it does for someone who may be taking only, say, 500 mg per day. 

How long you’ve taken the drug 

If you’ve been on metformin for 10 years and stop taking it, the drug may stay in your system for longer than it does for someone who took the medication for one week and then stopped. 

While medications take a while to build up in the system, they also take a while to break down, even if you’re not actively taking the medication. 

How long does it take metformin to get out of your system?

It takes approximately 5.5 times the elimination half-life for metformin to be completely cleared from your body which is:

5.5 x 17.6 hours = 96.8 hours 

That being said, it is completely fine (and expected) for you to take your metformin dose more often than every 4 days, but this is how long it would take to completely clear from your body if your kidneys are functioning properly. 

Different systems in the body may clear metformin faster than others, but within 4 days, you will have a negligible amount of metformin left in your body. 

Frequently asked questions

I forgot to bring my metformin on vacation. How long will it last? 

Metformin will be active in your body for 4 days, but you may notice higher blood sugar levels within 1 or 2 days of a missed dose.

Take your next recommended dose as soon as you can, but never “stack” your doses to make up for missed days. 

Can I mix metformin with other medications?

Metformin can negatively interact with other prescribed medications, so always tell your doctor about all medications you currently take if you’ve recently been prescribed metformin. If you’re on metformin, make sure you tell your doctor before starting any new medications.

Metformin can interact with:

• insulin
• sulfonylureas chlorpropamide (brand name Diabinese) 
• glimepiride (Amaryl)
• glipizide (Glucotrol)
• glyburide (DiaBeta, Glynase, Micronase)
• tolazamide (Tolinase)
• meglitinides (repaglinide [Prandin] and nateglinide [Starlix])

It may also interact with diuretics, steroids, and corticosteroids.

It can sometimes interact with substances in such a way that the risk of lactic acidosis (which, as noted earlier, can be fatal) is increased.

Your doctor may want you to completely be weaned off metformin before starting another drug.