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Diabetic patients taking the natural product goldenseal while taking the prescription drug metformin may be unwittingly sabotaging their efforts to maintain healthy blood glucose levels. Metformin — the world’s most-prescribed oral glucose-lowering medication — was included in a cocktail of selected drugs given to participants in a clinical study. The study found that after six days of taking goldenseal, participants had about 25 percent less metformin in their bodies.