Street heroin is widely varying and unpredictable in purity. This means that the user may prepare what they consider to be a moderate dose while actually taking far more than intended. If this occurs and the user takes a dose comparable to their previous use, the user may experience drug effects that are much greater than expected, potentially resulting in a dangerous overdose. One of the most important facts about heroin is that using this drug inevitably leads to drug addiction. Heroin addiction is characterized by compulsive drug seeking and use, and by neurochemical and molecular changes in the user’s brain. Heroin also produces profound degrees of tolerance and physical dependence, which are also powerful motivating factors for compulsive use and abuse. By the turn of the twentieth century, the addictive properties of cocaine had become clear, and the problem of cocaine abuse began to capture public attention in the United States. Small clandestine labs use any of hundred of different recipes to manufacture meth. Because the recipe using anhydrous ammonia was developed by the Germans during WWII, it is called the "Nazi" method of meth production. | 
