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Metformin

Brands and Forms

  • Fortamet
    • extended-release tablets: 500mg, 1000mg
  • Glucophage
    • tablets: 500mg, 850mg, 1000mg
  • Glucophage XR
    • extended-release tablets: 500mg, 750mg
  • Glumetza
    • extended-release tablets: 500mg, 1000mg
  • Riomet
    • oral solution: 500 mg per 5 mL

Uses of Metformin

Metformin is widely used in general medicine for normalizing blood sugar levels in people with Type II Diabetes Mellitus.

In psychiatry, it can be used to prevent or reverse the weight gain and metabolic side-effects (elevate glucose, lipids and cholesterol) that can be caused by atypical antipsychotics [ref, ref, ref].

How Metformin Works

Metformin is a hypoglycemic. It decreases blood glucose by decreasing hepatic glucose production, and may also decrease intestinal absorption of glucose and increase response to insulin.

Cautions when Using Metformin

In rare cases, lactic acidosis can occur as a result of Metformin accumulation, which can be caused by kidney function impairment or by the concurrent use of other medications that alter kidney functions. Impaired kidney function can be caused by kidney disease, cardiovascular disease, and advanced age. When lactic acidosis occurs, it is fatal in approximately 50% of cases. Kidney function tests should thus be measured prior to starting Metformin, and Metformin use should be avoided if kidney impairment is present.

Dosing of Metformin

The instant release forms of Metformin (Glucophage, Riomet) can be started at 500mg twice a day, or 850mg once daily, given with meals. Dose can be increased by 500mg every week or 850mg every two weeks; dose can also be increased from 500mg twice a day to 850mg twice a day after two weeks. The maximum dose is 2550mg/day. Doses above 2000mg are better tolerated when given 3 times a day with meals.

Fortamet can be started at 1000mg/day, given once daily with the evening meal, and increased by 500mg every week to a maximum of 2500mg/day.

Glumetza can be started at 1000mg/day, given once daily with the evening meal, and increased by 500mg every week to a maximum of 2000mg/day. Giving the larger doses twice daily may produce a better therapeutic effect.

Glucophage XR can be started at 500mg once daily with evening meal, and increased by 500mg every week to a maximum of 2500mg/day. Giving the larger doses twice daily may produce a better therapeutic effect.

Onset of action

When used to prevent weight gain or metabolic side-effects caused by atypical antipsychotics, the goal is for these side-effects not to occur while the Metformin is being used. If they do start to appear, the Metformin should be increased as needed according to the dosing schedule above.

When used to reverse the weight gain or metabolic side-effects caused by atypical antipsychotics, dose should be increased higher if reductions in these side-effects do not occur after two weeks at a lower dose.

Kidney impairment

Avoid using Metformin in these cases given the risk of lactic acidosis.

Liver impairment

Avoid using Metformin in these cases.

Use in Elderly

Ensure that kidney functions are normal before starting Metformin.

Side-effects of Metformin

Below is a list of most of the reported side-effects of Metformin. Most of these side-effects occur in only a minority of individuals, and many also resolve with time while the medication is continued.

Cardiovascular: palpitations.

Central Nervous System: weakness; headache; dizziness; light-headedness.

Dermatologic: increased sweating; rash; nail disorder.

Endocrine/Metabolic: low blood sugar.

Eyes, Ears, Nose and Throat: rhinitis; taste changes.

Gastrointestinal: diarrhea; nausea; vomiting; flatulence; indigestion; abdominal discomfort or pain; dyspepsia; constipation; distended abdomen; heartburn; taste disturbance.

Hematologic: increased rate of infections.

Muskuloskeletal: muscle pains.

Respiratory: shortness of breath; upper respiratory tract infection.

Common side-effects of Metformin

Diarrhea, nausea, vomiting, flatulence, indigestion, muscle weakness.

Rare but serious side-effects of note of Metformin

Lactic acidosis can occur in rare cases with accumulation of Metformin in the body.

Metformin overdose

Can cause lactic acidosis, which is usually associated with abdominal distress, increased somnolence, malaise, muscle pains, and respiratory distress. Can be fatal.

Metformin and pregnancy

Category B: animal studies have not shown any adverse effects at very high doses, but no controlled human studies have been done; should only be used in pregnancy if clearly needed and if benefits outweigh potential risks.

Medical Monitoring for Metformin

  • Monitor renal functions before starting the medication, and annually thereafter. In elderly individuals, it is preferable to monitor renal functions more frequently.
  • Vitamin B12 levels should be measured before starting the medication and then annually thereafter.

Drug Interactions with Metformin

  • Metformin blood concentrations can be increased by Amiloride, Cimetidine, Digoxin, Furosemide, Quinidine, Nifedipine, and Topiramate.
  • Alcohol use can increase the effects of Metformin on lactate metabolism, increasing the risk of lactic acidosis.
  • Metformin should not be used within 48 hours of iodinated contrast media are given
  • Metformin may lower Vitamin B12 levels.
  • Metformin can lower blood concentrations of Glyburide.