Just posted this in the Perc/Hydro thread, it belongs here. It is safe to snort Vics/Lortabs, but it is also stupid... as Phreex can tell you...
Hmm, info update:
First, acetaminophen is only hepatotoxic in large acute doses. There are a couple metabolic pathways, one (actually 2, sulfation/glucuronidation) is saturable, the other (2: via CYP2E1/CYP1A2) produces an intermediate (NAPQI). You take too much, the good pathway is saturated, the bad one starts running, eventually gluthianone stores run out, then the NAPQI turns into toxic metabolites which damage the liver.
Now the minimum toxic dose seems to be 150mg/kg. For me (180lb) this is 12 grams. Less than that at once, no problem. This is much lower if you are consuming alcohol, fasting, using drugs that inhibit P450 enzymes, etc. 4g seems to be the medical acute maximum OK dose in these cases.
Chronic use is typically recommended to be under 4-6 grams/day due to possible slow gluthianone depletion; and the fact that acetaminophen is a nonselective COX inhibitor -- COX-2 has important cytoprotective effects.
So: shooting acetaminophen is no problem; below the threshold toxic dose, no toxicity occurs. Hell, it is absorbed relatively quickly anyways. And doctors obviously have no problem with this: some of the subjects below received 2g IV!
Rawlins MD, Henderson DB, Hijab AR.
Pharmacokinetics of paracetamol (acetaminophen) after intravenous and oral administration.
Eur J Clin Pharmacol 1977 Apr 20;11(4):283-6