A. B. and C. - S1-E3
Corrected entry: When he breaks into the secret lab, Number 6 finds the syringe and substitutes plain tap water for the drug to be used on him. But even a small injection of unpurified, non-saline water can cause severe pain and is potentially lethal: it can cause oxygen bubbles to form in the bloodstream, and if this air embolism reaches the heart, cardiac arrest can result. Certainly not a risk Number 6 would take. (See http://en.wikipedia.org/wiki/Air_embolism.)
Correction: In order to produce a dangerous embolism, between 300 and 500 ml of air needs to be injected. That is way beyond the capacity of the small syringe we see. And Number 6 would be confident that, like all medical professionals, the lab technician would expel any air from the syringe before the injection. It's a bog standard medical procedure.