A device for injecting fluid from a pre-filled cartridge is provided. The device includes a needle for piercing a patient. After an injection, the needle can be retracted to prevent inadvertent contact with the contaminated needle. A needle retainer releasably retains the needle in the retracted position. The needle can subsequently be re-extended to administer a subsequent injection. After the cartridge is emptied, the cartridge can be removed, if desired, and replaced with another cartridge for additional injections. Alternatively, the needle can be substantially permanently retracted into the device and safely disposed of in a sharps container.