Abstract: Apparatus and a method are presented for delivering a therapeutic substance to a subject. A therapeutic substance delivery device includes an ultraviolet radiation source that disinfects a disinfection chamber of the therapeutic device. Control circuitry (a) activates the ultraviolet radiation source to irradiate the disinfection chamber, (b) measures a current input to the ultraviolet radiation source, the current being indicative of intensity of the ultraviolet radiation within the disinfection chamber, and (c) terminates the activation of the ultraviolet radiation source at a time determined based on the measured current. Other embodiments are also described.