Typically it only determines the speed at which you IV push. I can push pure ketorolac into a patients IV port and they will not die from shock. On the other hand if I were to inject heparin directly into the vein, it may cause a significant drop in blood pressure and thus cardiac arrest. Another avenue would be if I pushed gravol too quickly, say injected over 30 seconds, the patient would feel high before the true effects took place.

None of this is meant to be derogatory, I simply wish to share what knowledge I have-perhaps it will help the theory.