The purpose of this study was to examine whether the over-the-counter herbal medicinal plant St. John's wort affects glucose tolerance in healthy men. To do this, we included 10 healthy men who were examined by a 2-hr oral glucose tolerance test on three occasions: A: baseline; B: after 21 days of treatment with St. John's wort; and C: at least 6 weeks after the last capsule of St. John's wort was ingested. Plasma glucose, serum insulin and C-peptide levels were measured during an oral glucose tolerance test and used for estimation of area under the concentration-time curve (AUC) as well as indices of insulin sensitivity and insulin secretion. We found that treatment with St. John's wort increased total and incremental glucose AUC and 2-hr plasma glucose levels. Surprisingly, this effect was sustained and even further increased 6 weeks after the last capsule of St. John's wort was taken. No effect on indices of insulin sensitivity was seen, but indices of insulin secretion were reduced even after adjustment for insulin sensitivity. In conclusion, this study indicates that long-term treatment with St. John's wort may impair glucose tolerance by reducing insulin secretion in young, healthy men. The unregulated use of this over-the-counter drug might be a risk factor for impaired glucose tolerance and type 2 diabetes.
© 2015 Nordic Association for the Publication of BCPT (former Nordic Pharmacological Society).