 FACT: When Caine injects himself with the fluid to make him  invisible, you see bubbles going through the tube into his arm.  These bubbles, once in his veins, would cause severe pain (though  are probably not enough to kill him). Hence doctors in real life  flick the bubbles to the top of syringe and squirt them out. At the  least, a medical professional injecting air would be highly  unprofessional.