Sure, Mr. Whip, don't worry about them. It really is long odds you'll be hurt. But for fucks' sake why would you do it on purpose.
Anyway, if you need proof that it's possible to be grievously injured or killed from this,
unrelated to syringe IV of drugs, but in a hospital IV during catheterization or surgery, just search pubmed for "venous air embolism". I'll admit, finding a case study of death explicitly due to accidental air during user-IV is tough to find in that pile, or at least takes more than five minutes.
I got distracted by other case studies:
In the fourteen cases of central embolism of needle fragments following intravenous drug use, needles embolized to the right heart in six patients (43%), to the lungs in seven patients (50%) and to both right heart and lung in one patient (7%). In cases of pulmonary embolism, the right lung was affected in seven patients (88%) with only one case of left lung involvement. Followup ranged from nil to three years. Needle emboli were found at autopsy in one case and were not contributory to the cause of death. Of the seven cases of needle embolization to the heart, three (43%) resulted in complications requiring intervention: two patients developed pericarditis, and one developed consequent endocarditis with multiple septic emboli. Of the eight cases of pulmonary needle embolism, there were no documented complications or ongoing symptomology.
Of the published cases of centrally embolized foreign bodies, most have been iatrogenic complications of vascular access, intravascular monitoring, or diagnostic device usage (12); needle embolism from intravenous drug use would seem to be comparatively infrequent. However, due to lack of physician awareness of this entity, its inconspicuity on imaging, and its routine absence of symptoms, the true incidence of IVDU-related central needle embolism may be underestimated.
So why do I bring up needles? For one, if a needle can travel from your leg veins and lodge in your lungs, a tiny bubble can.
Plus, as with the above, the true incidence is unknown. If you only have intense pain in your chest for a while that eventually goes away, you're an IV drug user without insurance, no one checks, no one tells you, you never know you had a resolving embolism. If they find you dead somewhere, do you think they check for that sort of thing?
And, look, all but a couple people were sort-of fine! Nobody died! So it's safe, right? You'd totally break a needle off into your veins so you don't miss your shot.
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