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Glucophage®
Metformin is a drug used to lower blood sugar levels in people with non-insulin-dependent (type 2) diabetes.
Herbs
Summary of Interactions for Metformin
Depletion or interference Folic acid*
Vitamin B12
Adverse interaction Ginkgo biloba
Side effect reduction/prevention Calcium
Supportive interaction None known
Reduced drug absorption/bioavailability Guar gum*
Other (see text) DHEA
Magnesium
For the convenience of the reader, the information in the summary is categorized as follows: “Depletion or interference” indicates the drug may deplete or interfere with the absorption or function of the supplement or herb. “Adverse interaction” indicates that the supplement or herb used together with the drug may result in undesirable effects. “Side effect reduction/prevention” indicates the supplement or herb may reduce the likelihood and/or severity of a potential side effect caused by the drug. “Supportive interaction” indicates the supplement or herb may support or aid the function of the drug. “Reduced drug absorption/bioavailability” indicates that the supplement or herb may decrease the absorption and/or activity of the drug in the body. An asterisk (*) next to an item in the summary indicates that the interaction is supported only by weak, fragmentary, and/or contradictory scientific evidence.
Interactions with Supplements
Dehydroepiandrosterone (DHEA)
Metformin has been reported to increase blood levels of DHEA-sulfate in at least two studies. 1, 2
Folic acid and Vitamin B12
Metformin therapy has been shown to deplete vitamin B12 and sometimes, but not always, 3 folic acid as well. 4 This depletion occurs through the interruption of a calcium-dependent mechanism. Supplementation with calcium has reversed this effect in a clinical trial. 5 People taking metformin should supplement vitamin B12 and folic acid or ask their doctor to monitor folic acid and vitamin B12 levels.
Magnesium
In a study of patients with poorly controlled type 2 diabetes, low blood levels of magnesium, and high urine magnesium loss, metformin therapy was associated with reduced urinary magnesium losses but no change in low blood levels of magnesium. 6 Whether this interaction has clinical importance remains unclear.
Guar gum
In a small, controlled study, guar gum plus metformin slowed the rate of metformin absorption. 7 In people with diabetes this interaction could reduce the blood sugar-lowering effectiveness of metformin. Until more is known, metformin should be taken two hours before or two hours after guar gum-containing supplements. It remains unclear whether the small amounts of guar gum found in many processed foods is enough to significantly affect metformin absorption.
Interactions with Herbs
Ginkgo biloba
In a preliminary trial, administration of Ginkgo biloba extract (120 mg per day) for three months to patients with type 2 diabetes who were taking oral anti-diabetes medication resulted in a significant worsening of glucose tolerance. Ginkgo did not impair glucose tolerance in individuals whose diabetes was controlled by diet. 8 Individuals taking oral anti-diabetes medication should consult a doctor before taking Ginkgo biloba.
Other Interactions
Food
Food interferes with metformin absorption. 9, 10, 11 Taking metformin with food can reduce the absorption of the drug. Therefore, metformin should be taken an hour before or two hours after a meal unless stomach upset occurs.
Alcohol
Lactic acidosis is a rare but serious side effect of metformin. Alcohol increases the production of lactic acid caused by metformin, increasing the risk of lactic acidosis. 12 People taking metformin should avoid alcohol or consult with their doctor before consuming alcohol.