A dosage tracking system for a pen injector (440, 1940), the injector including a housing, a dosage selector (444, 1944) for adjusting a dosage and an indicator for displaying the dosage to the user, the system comprising a tracking device (401a, 1201a, 1201b) comprising: one or more motion sensors (1202, 2102) configured to sense the adjusting of the dosage of said injector; said one or more motion sensors configured to produce: a) a first output in response to rotating said dosage selector to add one or more units to a dosage of the injector, and b) a second output in response to rotating said dosage selector to subtract one or more units from a dosage of the injector; a processor (306, 2106) connected by a data transfer channel to said sensors for receiving an output of said sensors and configured to increment an estimated dosage in response to said first output, and decrement said estimated dosage in response to said second output; a display (414a, 1214, 1914) for viewing said estimated dosage; a viewport sized to display the dosage indicator (442, 1942) of the injector through said viewport; and a connector (336, 636) configured to reversibly hold the tracking device to the housing; wherein said viewport and a surface tangent at the center of said display are positioned at an angle between 0 to 45 degrees relative to each other, to allow simultaneous visualization of the dosage indicator through said viewport and said estimated dosage on said display from a single vantage point. one or more motion sensors (1202, 2102) configured to sense the adjusting of the dosage of said injector; said one or more motion sensors configured to produce: a) a first output in response to rotating said dosage selector to add one or more units to a dosage of the injector, and b) a second output in response to rotating said dosage selector to subtract one or more units from a dosage of the injector; a processor (306, 2106) connected by a data transfer channel to said sensors for receiving an output of said sensors and configured to increment an estimated dosage in response to said first output, and decrement said estimated dosage in response to said second output; a) a first output in response to rotating said dosage selector to add one or more units to a dosage of the injector, and b) a second output in response to rotating said dosage selector to subtract one or more units from a dosage of the injector; a processor (306, 2106) connected by a data transfer channel to said sensors for receiving an output of said sensors and configured to increment an estimated dosage in response to said first output, and decrement said estimated dosage in response to said second output; a display (414a, 1214, 1914) for viewing said estimated dosage; a viewport sized to display the dosage indicator (442, 1942) of the injector through said viewport; and a connector (336, 636) configured to reversibly hold the tracking device to the housing; wherein said viewport and a surface tangent at the center of said display are positioned at an angle between 0 to 45 degrees relative to each other, to allow simultaneous visualization of the dosage indicator through said viewport and said estimated dosage on said display from a single vantage point.