A parenteral injection device comprises a housing defining an internal cavity and an output, a first reservoir disposed within the internal cavity, where the first reservoir defines a first chamber that is configured to store a drug and is in fluid communication with the output, a first drive mechanism to selectively force the drug from the first chamber, and a second reservoir disposed within the internal cavity, wherein the second reservoir defines a second chamber that is configured to store a gas and is in fluid communication with the output. The injection device further comprises a second drive mechanism to selectively force the gas from the second chamber, and a controller in signal communication with the first and second drive mechanisms, such that the controller is configured to selectively direct the first and second drive mechanisms to force the drug and gas from the first and second chambers, respectively, to the output in a predetermined sequence.