Heroin is a processed form of morphine which produces a calming effect, reducing pain and giving users a rush within a few seconds of using it. Small doses give a general sense of well being and larger ones cause extreme relaxation. Most of the time heroin is injected but it can also be snorted, inhaled or smoked or swallowed. It is much more addictive than morphine and causes a rapid physical as well as psychological dependence. Tolerance also tends to build quickly, so users have to take larger amounts for the same effects, making overdosing an ever-increasing risk. Withdrawal and post effect "crashes" are very uncomfortable which also drives increased usage frequency and dosage. Injected heroin is often done in poor conditions or using shared needles (which brings its own risks in terms of HIV, hepatitis and blood poisoning) and damages veins quite severely, even causing gangrene. Heroin inhibits normal cough reflexes which can mean that users die by inhaling their own vomit. Overdose causes a coma and normally leads to death.