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Metformin vs. Lantus: Side Effects & Dosage for Diabetes Drugs

What is the difference between metformin vs. Lantus?

What is metformin? What is Lantus?

Metformin is an oral medication used to treat type 2 diabetes. Metformin works by increasing the sensitivity of liver, muscle, fat, and other tissues to the uptake and effects of insulin, which lowers blood glucose (sugar) levels. Metformin does not increase the concentration of insulin in the blood and does not cause low blood glucose levels (hypoglycemia) when used alone. Metformin can reduce complications of diabetes such as heart disease, blindness, and kidney disease. Metformin is also used to treat polycystic ovaries and weight gain due to medications used for treating psychoses.

Lantus (insulin glargine) is a man-made injectable form of long-acting insulin used to regulate sugar (glucose) levels in type 1 and type 2 diabetes. Individuals with type 1 diabetes do not produce insulin on their own; and individuals with type 2 diabetes do not produce enough insulin, or insulin is not as effective due to insulin resistance. Lantus works the same way as natural human insulin, but it's action lasts longer. It helps diabetic patients regulate glucose or sugar in the body. Lantus works by promoting movement of sugar from blood into body tissues and also stops sugar production in liver.

What are the side effects of metformin and Lantus?


The most common side effects with metformin are

These symptoms occur in one out of every three patients. These side effects may be severe enough to cause therapy to be discontinued in one out of every 20 patients. These side effects are related to the dose of the medication and may decrease if the dose is reduced.

Metformin may also cause:

A serious but rare side effect of metformin is lactic acidosis. Lactic acidosis occurs in one out of every 30,000 patients and is fatal in 50% of cases. The symptoms of lactic acidosis are

  • weakness,
  • trouble breathing,
  • abnormal heartbeats,
  • unusual muscle pain,
  • stomach discomfort,
  • light-headedness, and
  • feeling cold.

Patients at risk for lactic acidosis include those with reduced function of the


Common side effects of insulin glargine are:

  • Decreased blood sugar and injection site pain
  • Water retention in the joints and weight gain

Local allergic reactions that may occur at the injection sites are:

Long-term use of insulin glargine can lead to thickening of fat tissues at the injection site. Severe allergic reactions are:

  • Swelling under the skin
  • Very low blood pressure
  • Bronchospasm (tightening of chest that leads to difficulty breathing)

Individuals should contact a healthcare professional if they experience any of the above reactions.

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What is the dosage of metformin vs. Lantus?


  • For treating type 2 diabetes in adults, metformin (immediate release) usually is begun at a dose of 500 mg twice a day or 850 mg once daily. The dose is gradually increased by 500 mg weekly or 850 mg every two weeks as tolerated and based on the response of the levels of glucose in the blood. The maximum daily dose is 2550 mg given in three divided doses.
  • If extended tablets are used, the starting dose is 500 mg or 1000 mg daily with the evening meal. The dose can be increased by 500 mg weekly up to a maximum dose of 2000 mg except for Fortamet (2500 mg of Fortamet, once daily or in two divided doses). Glumetza tablets (500 -1000mg formulations are given once daily (either 1000 to 2000mg). Fortamet and Glumetza are modified release formulations of metformin. Metformin should be taken with meals.
  • For pediatric patients 10-16 years of age, the starting dose is 500 mg twice a day. The dose can be increased by 500 mg weekly up to a maximum dose of 2000 mg in divided doses.
  • Children older than 17 years of age may receive 500 mg of extended-release tablets daily up to a maximum dose of 2000 mg daily. Extended-release tablets are not approved for children younger than 17 years of age.
  • Metformin-containing drugs may be safely used in patients with mild to moderate renal impairment. Renal function should be assessed before starting treatment and at least yearly.
  • Metformin should not be used by patients with an estimated glomerular filtration rate (eGFR) below 30 mL/minute/1.73 m2 and starting metformin in patients with an eGFR between 30-45 mL/minute/1.73 m2 is not recommended.
  • Metformin should be stopped at the time of or before administering iodinated contrast in patients with an eGFR between 30 and 60 mL/minute/1.73 m2; in patients with a history of liver disease, alcoholism, or heart failure; or in patients who will be administered intra-arterial iodinated contrast. Kidney function should be evaluated 48 hours after receiving contrast and metformin may be restarted if kidney function is stable.


  • The starting dose should be individualized based on the type of diabetes and whether the patient is insulin-naïve
  • Administer subcutaneously once daily at any time of day, but at the same time every day.
  • Rotate injection sites within an injection area (abdomen, thigh, or deltoid) to reduce the risk of lipodystrophy.
  • Converting from other insulin therapies may require adjustment of timing and dose of Lantus. Closely monitor glucose especially upon converting to Lantus and during the initial weeks thereafter.


______________ is another term for type 2 diabetes.
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What drugs interact with metformin and Lantus?


  • Cimetidine (Tagamet), by decreasing the elimination of metformin from the body, can increase the amount of metformin in the blood by 40%. This may increase the frequency of side effects from metformin.
  • Ioversol (Optiray) and other iodinated contrast media may reduce kidney function, which reduces elimination of metformin, leading to increased concentrations of metformin in the blood. Metformin should be stopped 48 hours before and after use of contrast media.
  • Thiazide diuretics, steroids, estrogens, and oral contraceptives may increase blood glucose and reduce the effect of metformin. When these drugs are stopped, patients should be closely observed for signs of low blood glucose.
  • Alcohol consumption increases the effect of metformin on lactate production, increasing the risk of lactic acidosis.


There are many drugs that do not directly interfere with insulin glargine, but they may affect glucose breakdown in the body. This necessitates adjustments of insulin glargine doses.

Pramlintide (Symlin), disopyramide (Norpace), fenofibrate (Tricor), fluoxetine (Prozac), and aspirin can increase blood-sugar-lowering effect of insulin glargine.

Other drugs that can decrease the blood-sugar-lowering effect of insulin glargine are:

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Are metformin or Lantus safe to use while pregnant or breastfeeding?


  • There are no adequate studies in pregnant women. Most experts agree that insulin is the best treatment for pregnant women with diabetes.
  • Metformin is excreted into breast milk and can, therefore, be transferred to the nursing infant. Nursing mothers should not use metformin.


  • Safe and effective use of insulin glargine is not established for pregnant females.
  • It is not known whether insulin glargine enters breast milk; therefore, it should be used with caution in females who are breastfeeding.

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