Dietary fibre and biproducts of some vegetable extracts (Guar etc.) are advocated to reduce postprandial hyperglycaemia but there is some evidence that these may disturb the absorption kinetics of orally administered drugs. The effect of Guar gum on the digestive absorption of metformin was tested in 6 healthy subjects given 1700 mg of metformin with a standardised meal. Metformin blood levels showed that when given together with Guar there was a reduction in the absorption rate over the first 6 hours. These findings suggest that combination therapy may diminish the anti-hyperglycaemic action of metformin.