A wearable emergency drug injection device for injecting a substance into a patient, comprising: a housing defining at least a first chamber (122,322,422,622,722) and a second chamber (124,324,424,624); an injector disposed within the housing and comprising: a hollow needle (152,352,452,652,752) corresponding to a first piston (132,332,432,632,732) initially positioned at one end of the chamber opposite an opening, the first piston disposed and translatable within the first chamber towards a first end of the first chamber, the first piston defining a void intersecting a hollow portion of the hollow needle; and a first propellant disposed within the first chamber and positioned to force the first piston towards the first end of the first chamber in response to activation of the first propellant, and a dispenser disposed within the housing and comprising: a second piston (134,334,434,634) initially positioned at one end of the chamber opposite an opening and disposed and translatable within the second chamber towards a first end of the second chamber; a second propellant disposed within the second chamber and positioned to force the second piston towards the first end of the second chamber in response to activation of the second propellant; wherein translation of the first piston to the first end of the first chamber exposes the void to the first end of the second chamber. a housing defining at least a first chamber (122,322,422,622,722) and a second chamber (124,324,424,624); an injector disposed within the housing and comprising: a hollow needle (152,352,452,652,752) corresponding to a first piston (132,332,432,632,732) initially positioned at one end of the chamber opposite an opening, the first piston disposed and translatable within the first chamber towards a first end of the first chamber, the first piston defining a void intersecting a hollow portion of the hollow needle; and a first propellant disposed within the first chamber and positioned to force the first piston towards the first end of the first chamber in response to activation of the first propellant, and a dispenser disposed within the housing and comprising: a second piston (134,334,434,634) initially positioned at one end of the chamber opposite an opening and disposed and translatable within the second chamber towards a first end of the second chamber; a second propellant disposed within the second chamber and positioned to force the second piston towards the first end of the second chamber in response to activation of the second propellant; wherein translation of the first piston to the first end of the first chamber exposes the void to the first end of the second chamber. a hollow needle (152,352,452,652,752) corresponding to a first piston (132,332,432,632,732) initially positioned at one end of the chamber opposite an opening, the first piston disposed and translatable within the first chamber towards a first end of the first chamber, the first piston defining a void intersecting a hollow portion of the hollow needle; and a first propellant disposed within the first chamber and positioned to force the first piston towards the first end of the first chamber in response to activation of the first propellant, and a dispenser disposed within the housing and comprising: a second piston (134,334,434,634) initially positioned at one end of the chamber opposite an opening and disposed and translatable within the second chamber towards a first end of the second chamber; a second propellant disposed within the second chamber and positioned to force the second piston towards the first end of the second chamber in response to activation of the second propellant; a second piston (134,334,434,634) initially positioned at one end of the chamber opposite an opening and disposed and translatable within the second chamber towards a first end of the second chamber; a second propellant disposed within the second chamber and positioned to force the second piston towards the first end of the second chamber in response to activation of the second propellant; wherein translation of the first piston to the first end of the first chamber exposes the void to the first end of the second chamber.