The main issue, to my understanding, is the tissue/vein irritation and necrosis.
As for why it causes those things, that's beyond my level of knowledge, and I don't have access to PubMed here. My guess would be a Ph problem, but that's entirely a guess.
Keep in mind, some of the confusion may stem from 2 similar drugs:
hydroxyzine
hydrochloride (atarax) which
is approved for IV use.
hydroxyzine
pamoate (vistaril) which is not approved for IV use.
I looked up Vistaril/Hydroxyzine in
Davis's Drug Guide for Nurses, 5th Edition, page 617:
"Hemolysis may result from IV injections." "Significant tissue damage, necrosis, and sloughing may result from IV injections."
Also, in
Mosby's Drug Guide for Nurses, 4th Edition, page 595:
"Never give IV, severe tissue necrosis can result."
From the manufacturer's drug label (
http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?id=2589)
Hydroxyzine Hydrochloride Injection, USP is a sterile aqueous solution intended for
intramuscular administration.
Hydroxyzine hydrochloride intramuscular solution is intended only for intramuscular administration and
should not, under any circumstances, be injected subcutaneously, intra-arterially or intravenously.
Based on all literature out there, the actual label on the vial (Bold red print,
"Not for IV use!), and standard practice, Vistaril should never be given IV. It's not that the drug manufacturers are trying to keep you from getting high; they just don't want your arm to fall off.
Be safe.