We introduce a mobile device application that displays key information about caffeine: the pharmacokinetics (time course of drug levels) and pharmacodynamics (the effects of caffeine level) visually on the iPhone, iPod Touch, and iPad. This application, Caffeine Zone, is based on an existing model of caffeine physiology using user inputs, including caffeine dose, start time, and consumption speed. It calculates the caffeine load in a user for the next twenty-four hours and displays it using a line chart. In addition, it shows whether the user is currently in the "cognitive alert zone" (the range of caffeine where a normal person might benefit most from caffeine) or the "possible sleep zone" (the range of caffeine where sleep is presumed not affected by caffeine level.) Understanding the pharmacokinetics and pharmacodynamics of caffeine can help people using caffeine to improve alertness, including in operational environments. Caffeine Zone may also help users create a mental model of caffeine levels when the device is not available. We argue that this app will both teach users the complex absorption/elimination process of caffeine and help monitor users' daily caffeine usage. The model, with additional validation, can be part of a system that predict cognitive state of users and provide assistances in critical conditions.