Heat Rushes are mostly attributed to salts...
Many cuts will contain some form of salt as the xtalling seed or a large part of the compound.
The cough Rush is usually attributed to excess solvent trapped in the xtal lattice. Most noticable is ether.
That is why you'll see recrystallization being preached so much around here.
Many impurities exist as well, depending on the synth route and each impart their own "Flavor/Buzz/Side-Effect".
JawClenching/TeethGrinding/MuscleCramping Dope is riddled with impurities but also will have that signature RUSH, which isn't ever synonymous with pure meth!
Pure Meth is a smoothe ride until you reach the higher doses, and then it's just real intense.
I've never heard of or witnessed a meth overdose causing instant death or locking up vital organs!
Coke can stop a heart.
Meth in very high doses will make a bee braindead before it causes organ flailure.
That is my assessment...don't take that to the bank...(or the grave!

)
While I'm sure it's a possibility, it takes much more than one would think or what many are told.
While niacin may be included in some streetbought gear...it would be more noticable than just a heat rush...
If there were alot of niacin, there would be a heavy skin and face flushing period followed by brief but intense itching IMHO at the flushing site.
Never IV'ed niacin but comsumed it orally in heavy doses.
And one more thing....alot of stereotyping surrounds IV'ing drugs. Provided the drug is in it's pure form, there is no cleaner route of adminstration. And Provided sanitary and sterile measures are practiced.
For those that IV any drug, they should research the term
"Harm Reduction" and learn the safety rules and proper practices. Never waiver from those guidelines, cause all it takes is one stupid mistake or over/undersight sacrificed to the "InstantGratification God" to make your life miserable.
Hospitals and qualified medical professionals wouldn't choose the IV/IM route of adminstration most often, if it weren't the safest/cleanest route of getting drugs into your ass!
Again....this is all my opinion and I'm no expert by any stretch of the imagination.
So take it with a large grain of sympathomimetic amine salt