i do the flick method of aspiration as well. but i've also pulled with 22 gaugers, and withdrew blood. whether the vein wall collapses or not depends on the size of the vein as well. another way to avoid intravenus injections, is to vary the depth of the injection as your pushing the oil out. go in all the way, aspirate, and inject a bit, pull out a bit aspirate again, and push more oil in and repeat. not only will this avoid the possibilities of IV injections, but it will spread the depot out, lessening the chance of infection (more perfusion/volume of oil), and shortening the half life (higher surface area/volume ratio of the depot). i pretty much practice this all the time.
injecting into a vein is bad for another reason besides the possibility of stroke. the drugs that go into the blood stream will be deesterized but esterases within hours, and you'll metabolize all the drug. you're blood levels of the hormone will soar for several hours, but they'll quickly come down, and you'll just be left with the oil that didn't go IV.