Your question is not clear! Once the dawn breaks, the adhan is called. Once it is called, you must not drink or eat and must fast whether you had prayed fajr or not.

The question is regarding fasting in Ramadan, I have severe acid reflux after eating, the food always refluxes, In this case, If I fast before offering fajr prayer, It may disturb my concentration in prayer, only for this condition, Can I fast as soon as completing fajr prayer?