A medication injection device comprising a medication container to contain medication； a connector provided with a blocking feature； a push rod having a plurality of teeth disposed along the length, and a spring biasing against the push rod to move proximally. The spring can be compression spring, extension spring, torsion spring, or constant force spring. By enabling or disabling engagement between the blocking feature on the connector and the tooth on the push rod, the push rod is either prevented from moving distally or allowed to move distally. The medication is drawn into the medication container when the push rod moves proximally and the medication is dispensed out of the medication container when the push rod moves distally.
