Does Metformin Cause Constipation? Gastroenterologist Explains.

Metformin can cause constipation in about 1% of patients (1). However, it is more common for metformin to cause diarrhea than constipation. Think of other causes of constipation, such as diabetes itself.

In this article, you will learn about the link between metformin and constipation.  Constipation during metformin treatment is not always due to the medication. Other co-existent conditions can lead to such problems. 

Does metformin cause constipation?

Metformin rarely leads to constipation. According to research, less than 1% of patients receiving metformin get constipated. In addition, constipation is known to cause diarrhea rather than constipation. 

The incidence of diarrhea with metformin is up to 53% of patients taking the immediate-release (IR) metformin. The incidence is much lower with the extended-release formula (about 10%) (2).

The table below illustrates the estimated frequencies of different gastrointestinal side effects of metformin (1, 2):

Constipation is less prevalent as a side effect of metformin. The frequency of constipation was about 1% in the extended-release (ER) metformin and about 0.6% in the immediate-release (IR) metformin (1).

Constipation is defined as having fewer than three bowel movements per week. Stools are often hard, dry, or lumpy. In addition, constipation is associated with other abdominal symptoms such as bloating and distension (mainly on the left side).

Having constipation during metformin therapy can be due to other causes (discussed below).

Suspect metformin as a cause of constipation if you begin to experience it shortly after taking metformin. 

If constipation has developed later after months or years of metformin, it is better to think of an alternative cause of constipation other than metformin.

What are the other possible causes of constipation with metformin?

Constipation is considered a rare side effect of metformin. So, it is better to think of other common causes. Having constipation while taking metformin for diabetes can arise from a variety of diabetes-related factors. 

It makes sense to think of alternative causes of constipation if it has developed after months or years of metformin treatment. 

Possible alternative causes of constipation during metformin include:

A. Diabetic neuropathy (diabetes-induced constipation).

A set of nerves controls your gut motility; they are called the ANS (autonomic nervous system). Long-lasting or uncontrolled diabetes can lead to nerve damage.

With the damage of the nerves controlling your bowel movements, bowel motility will become slower. Consequently, you will get constipation with diabetes. 

About 50% of patients with diabetes have nerve damage (diabetic neuropathy) (4). So, it makes sense to think of diabetes as the cause of your constipation rather than metformin. 

B. Low caloric intake and dehydration.

IF you’re taking metformin for diabetes, you probably follow a strict diet plan to control diabetes. However, low intake of overall calories and low fiber intake is a known risk factor for constipation (5). 

Moreover, if your diabetes is out of control, you may experience increased urine amounts. Frequent urination ultimately leads to dehydration with is another risk factor for constipation.

C. Other Medications that you take with metformin.

Diabetic patients are always poly-medicated and may have other chronic diseases such as hypertension. In addition, many different medications are more likely to cause constipation than metformin.

Review your list of medications for:

• Antihypertensives such as Calcium Channel Blockers (as amlodipine).
• Antidepressants that are used for diabetic nephropathy. 
• Analgesics (most of the NSAIDs can lead to constipation).
• Antacids.
• Anti-spasmodics.
• Iron supplements.
• Antihistamines.

The complete list of medications that can cause constipation is illustrated in the table in the next section.

D. Others.

Always seek medical advice if you suffer from unexplained constipation. Too many conditions and diseases can lead to constipation while taking metformin.

Don’t stop metformin because of constipation without medical permission from your doctor. This can lead to worsening diabetes (if you are taking it for diabetes mellitus).

A thorough evaluation by your doctor for the cause of constipation is the best way to figure out its relation to metformin. 

Possible other causes of constipation are summarized in the table below (6)