Apparatus is provided that includes a drug capsule containing an oral drug, and a sensing apparatus that includes a housing and a sensor. The housing is shaped so as to define exactly one hemispherical portion and exactly one cylindrical portion, which together define an internal surface tightly fitted to at least a portion of an external surface of the drug capsule. The sensor includes first and second electrodes, which comprise first and second electrode surfaces, respectively； and circuitry, which (a) is attached to the housing, (b) is electrically coupled to the first and the second electrode surfaces, and (c) is configured to drive a current between the first and the second electrode surfaces. Other embodiments are also described.
