A mechanical patient-controlled analgesia device for injecting medicine into a patient's IV, includes an IV reservoir and a syringe communicated to the IV reservoir and having a syringe plunger. A mechanical biasing element is provided for exerting bias on the syringe plunger in a first syringe-filling direction to draw medicine from the IV reservoir into the syringe. An injection plunger is operable by the patient to move the syringe plunger in a second injection direction against the bias to inject the medicine in the syringe into a patient's IV. The injection plunger is movable independently of the syringe plunger and is engaged with but unconnected to the syringe plunger when the patient moves the injection plunger to inject the medicine. The patient can inject only a calibrated amount of medicine drawn into the syringe as determined by calibration of the biasing element and a metering element for a given viscosity of the medication.