Heroin is not a product which is naturally produced or desired by the human body. The physical and mental reactions to the drug are similar to an allergic reaction with potentially deadly consequences. It is possible to be allergic to anything that the body does not recognize as being usable or beneficial.
yes, it is possible to be allergic to anything, and heroin, just like any thing else has chemicals, ingredients, mixtures, all of which can affect you if you are unfamiliar with them. it is not very common, and is not something you can really be tested for ahead of time i do not think.
You can be allergic to anything, including heroin. Essentially, heroin is just melted down morphine, which most people aren't allergic to, but can be. Heroin isn't something that people should do anyways, so allergic or not, it is best to just stay away, because being allergic to it is the least of most people's concerns.