On not all, but the majority of my shots of heroin, I also get a histamine reaction. (OP, this is exactly what is happening to you from the way you described it). I am very sensitive to a lot of things, not just heroin. A certain type of mosquito that lives in the South will bite me and cause the same reaction but also cause whatever limb it bit to swell tremendously, especially if it happens to bite me on a spot near a vein. I have had my foot, hand and ankle so severely swollen that they hurt (cause of the pressure of the swelling on everything, skin, nerves etc) from a simple mosquito bite and it lasts for a couple of days.
The point is it is a histamine reaction. I personally get them a good bit of the time from IV heroin even when I dont miss. I get the symptoms (swelling, redness, hives, itching, pain) right at the injection site along with sometimes all along the path of that vein in my arm. Some people are just more sensitive than others. BTW, even out of the same batch of H I can get varying degrees of the histamine reaction at different times. I may do one injection in the morning and have no reaction then do another later that same day with the exact same heroin and get a really bad histamine reaction. Not sure why it varies but it is not extremely uncommon to get histamine reactions from dope.
