An auto injector for preparing and/or administering a medicament from a cartridge having a first cartridge end and a second cartridge end and having a cartridge outlet at the first cartridge end , the cartridge comprising a cartridge compartment with a first cartridge subcompartment containing a first medicament component of the medicament and a second cartridge subcompartment containing a second medicament component of the medicament, the auto injector comprising: a housing ; a cartridge receiver with a cartridge receiver opening , the cartridge receiver being configured to receive the cartridge by insertion of the second end of the cartridge through the cartridge receiver opening ; a cartridge sensor configured to detect reception of the cartridge in the cartridge receiver and provide a cartridge detection signal indicative of the cartridge being received in the cartridge receiver ; a drive module coupled to move a plunger rod between a retracted plunger rod position (PR) and an extended plunger rod position (PE), the plunger rod being configured to move a first stopper ; an orientation sensor configured to provide an orientation signal indicative of the orientation of the cartridge when received in the cartridge receiver ; a processing unit coupled to the orientation sensor , the cartridge sensor , and to the drive module ; characterized in that the processing unit is further being configured to: - receive a first input signal; - receive the cartridge detection signal; - receive the orientation signal; - control the drive module to move the plunger rod to a mix plunger rod position following reception of the cartridge detection signal and the first input signal; and - control the drive module to move the plunger rod to a prime plunger rod position following completion of movement of the plunger rod to the mix plunger rod position, wherein control of the drive module to move the plunger rod to the prime plunger rod position is based on the orientation signal. a housing ; a cartridge receiver with a cartridge receiver opening , the cartridge receiver being configured to receive the cartridge by insertion of the second end of the cartridge through the cartridge receiver opening ; a cartridge sensor configured to detect reception of the cartridge in the cartridge receiver and provide a cartridge detection signal indicative of the cartridge being received in the cartridge receiver ; a drive module coupled to move a plunger rod between a retracted plunger rod position (PR) and an extended plunger rod position (PE), the plunger rod being configured to move a first stopper ; an orientation sensor configured to provide an orientation signal indicative of the orientation of the cartridge when received in the cartridge receiver ; a processing unit coupled to the orientation sensor , the cartridge sensor , and to the drive module ; - receive a first input signal; - receive the cartridge detection signal; - receive the orientation signal; - control the drive module to move the plunger rod to a mix plunger rod position following reception of the cartridge detection signal and the first input signal; and - control the drive module to move the plunger rod to a prime plunger rod position following completion of movement of the plunger rod to the mix plunger rod position, wherein control of the drive module to move the plunger rod to the prime plunger rod position is based on the orientation signal. <EOS>