A medical module comprising: a primary module housing extending along a first longitudinal axis from a first module housing end to a second module end; a secondary module housing coupled to the primary module housing, the secondary module housing including first and second extensions (130, 134) extending along and partially circumscribing a second longitudinal axis generally parallel to the first longitudinal axis to define at least a portion of a hollow bore, the at least a portion of a hollow bore configured for attachment over an actuation unit of a drug delivery pen; a dosage sensor coupled to the primary module housing; a follower portion configured to engage a dosage selector of the drug delivery pen and physically connected to the dosage sensor, the follower portion being disposed for movement relative to the primary module housing along the first longitudinal axis as dosage is adjusted, wherein the dosage selector is rotational to allow for axial motion of the dosage selector to select a dose; locator forks (152a, 152b) connected to the follower portion, the locator forks (152a, 152b) being configured to capture an actuation button of the drug delivery pen between the locator forks and coupled to the dosage selector such that the locator forks (152a, 152b) with follower portion move longitudinally along the first longitudinal axis, following the axial movement of the dosage selector or the actuation button along the second longitudinal axis; a power source coupled to the primary module housing; and a microcontroller disposed proximate the primary module housing and electrically coupled to both the dosage sensor and the power source for receiving data on the position of the dosage selector and actuation button such that the amount of drug injected via the drug delivery pen can be measured. a primary module housing extending along a first longitudinal axis from a first module housing end to a second module end; a secondary module housing coupled to the primary module housing, the secondary module housing including first and second extensions (130, 134) extending along and partially circumscribing a second longitudinal axis generally parallel to the first longitudinal axis to define at least a portion of a hollow bore, the at least a portion of a hollow bore configured for attachment over an actuation unit of a drug delivery pen; a dosage sensor coupled to the primary module housing; a follower portion configured to engage a dosage selector of the drug delivery pen and physically connected to the dosage sensor, the follower portion being disposed for movement relative to the primary module housing along the first longitudinal axis as dosage is adjusted, wherein the dosage selector is rotational to allow for axial motion of the dosage selector to select a dose; locator forks (152a, 152b) connected to the follower portion, the locator forks (152a, 152b) being configured to capture an actuation button of the drug delivery pen between the locator forks and coupled to the dosage selector such that the locator forks (152a, 152b) with follower portion move longitudinally along the first longitudinal axis, following the axial movement of the dosage selector or the actuation button along the second longitudinal axis; a power source coupled to the primary module housing; and a microcontroller disposed proximate the primary module housing and electrically coupled to both the dosage sensor and the power source for receiving data on the position of the dosage selector and actuation button such that the amount of drug injected via the drug delivery pen can be measured.