…the time you should use a swab is BEFORE injecting, ideally about 10 seconds before (to allow the alcohol to evaporate) and with just a single, firm rub on the skin. So why not use it after? Well its because alcohol thins blood, if you put it on an open wound then you will promote rather than stem the bleeding. The more you bleed, the more you bruise. Also the more you bleed the more blood you have to cross contaminate with.