The monitoring breathalyzer has an alcohol sensor, a non-volatile memory, 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 the user's metabolism rate or based on a time to a predetermined calibration point from a drink start time.