The monitoring breathalyzer has an alcohol sensor, a processing unit or processor, and a screen. The processing unit determines the accuracy of the breathalyzer using the user's body as a simulator. In monitoring mode, the processing unit receives a BAC measurement from the alcohol sensor based on the breath sample provided by the user at a sample time and determines a reference point from the BAC measurement. The sample time is determined based on a time to a predetermined calibration point from a drink start time.