oh god I would not shoot up w/ 20g needles ...those are literally horse needles, and I should know as I've IVed a good bit w/ 20 or 22 gauge screw on needles and a 5ml syringe bought from Tractor Supply when I could not get rigs ...it always bled so much upon withdrawal I figured I was losing a good bit of the drug in that and often left bruises and swelling and I'm sure contributed to fucking up my veins. plus you can't get EVERYTHING out of a big syringe w/ a screw on pin like that unless you re-register after doing the shot and that's not good to do. never experienced having a whole couple units left in there so I can't speak to that, but I would advise you to if you can use something more in the realm of 28-31g to minimize damage.