It depends on the duration of action of the anti-diabetic drug. Drugs, which are short acting, need to be taken 10-20 minutes before the meals; most drugs act for 24 hours and may be taken even after meals. Many drugs are in combination and usually advised to be taken after meals. In fact, drugs are usually given after meals to avoid gastric irritation; otherwise, they could be taken even before meals. There is one group of drug called Alpha-glucosidase Inhibitors (AGIs) that have to be taken as first bite with food and never after food.