What is the active ingredient in heroin?

The "ingredients" in heroin is merely the drug, heroin (although its chemical name is diacetyl-morphine); the drug got its renown trade name (heroin) shortly after it's discovery/development and it has stuck (for example, in opposition we call morphine by its chemical name; which is morphine, and not by its trade name, or brand name it was sold under when it was first developed). In regards to other added substances in black-market heroin, this varies.. and they generally do not contribute to any of the psychoactive effects (unless an antihistamine is used as a cutting agent, this will cause added sedation and will minorly potentiate the opiate-effects). Quinine is also a common ingredient in black-market heroin. But in general, added substances to heroin vary greatly and are more likely to consist of "fillers" cheaply and readily available to the drug dealer and are therefore impossible to identify without a lab analysis.

But calling added substances "ingredients" is somewhat of a misnomer. They are only found in black-market grade heroin (and not "medical grade" heroin, as available in some western countries, although not in the U.S.); and can be seen more as a "contaminant" than an "ingredient", and in fact, a lot of these added substances and the impurity of black-market quality heroin contribute to its major health effects when injected. Since not all substances are meant for injection and can cause complications, or if handling of the drug caused bacterial contamination--regular intravenous use can lead to: stroke, heart attack, gangrene, limb amputation, infection, and/or organ damage (these would not be normal side effects if pure or medical grade heroin were being used, and injected in a safe fashion). To repeat myself, the only ingredient that is supposed to technically be in heroin; is diacteylmorphine.

In some instances, if heroin is improperly, inadequately, or poorly synthesized from morphine and opium some left over, unreacted opiate alkaloids or plant matter (originally found in opium, the base material from which heroin is made) may remain in the final product. And in some rare cases, black marketheroin will not actually contain diacetylmorphine at all, but will rather contain a different opiate or opiates. The most common of these substitutes are: morphine, the-more-potent drug fentanyl, other fentanyl derivatives (alfentanil, sufentanil, remifentanil, carfentanil), or the specific fenantyl derivative alpha-methylfentanyl which was the active ingredient in the drug sold as "china white heroin." Because most all opiates are cross tolerant with each other, the user will notice little to no difference in the substitution if the strength is kept relatively stable and if a short-acting opiate is used (if a longer acting opiate is used, the user will notice a less profound "rush"). And of course at some other times, the drug presented as "heroin" wont contain any diacetylmorphine or any opiate at all. (i.e., the user got ripped off!)