Duhrickjames, your statements are on the right track but I find them to be inversely exaggerated...if that is a valid expression. Street heroin is generally cut with a plethora of other substances depending on the area but most should be absent of strychnine or poisons of such toxicity. Dealers don't necessarily want to kill their customers, at least not so instantaneously.
More common cutting agents are lactose, mannitol, sugar, vitamin b12, talc powder etc. It is true these cuts can make use of street heroin harmful by precipitating collapsed veins and infections for intravenous users.
The user never really knows how pure their heroin is and there is also a risk of it containing other, more potent opioids such as fentanyl. If they had originally been using impure product and then receive some of higher-purity or which contains active cuts they might not bother to test it and instead go right to administering their 'normal' dose, risking an overdose.

Many people seem to believe heroin must injected. This is not true, it can be insufflated and smoked but shooting gives addicts the most bang for their buck. Heroins never made me 'crash' either, just itch and sleep.