If you inject heroin for an amount of time, your veins are likely to collapse, or ‘blow’. This means the veins become so swollen from the trauma of the needle, they create clotting inside the wall. The clots harden into scar tissue and the veins collapse inward, restricting blood flow until the vein closes completely and nothing can get through. Not only does this mean you have to find other veins to use to inject heroin (and as more veins collapse, this becomes more difficult), it can lead to medical problems such as deep vein thrombosis, impaired blood circulation, skin infections, pain and abscesses. The good news is that in most cases collapsed veins can be fixed. Many heal naturally over time while some do so after surgery. However, some may never recover and if a vein collapses twice due to persistent heroin use, it may never return to full function.