Since many individuals make up the distribution chain, heroin is "cut" to maximize profits; substitutions for cutting agents can cause significant danger. Black tar heroin is popular because of its high purity, cost, and availability. Many illegal aliens are involved in smuggling this drug from Mexico. Concerns about AIDS have caused users to switch from injecting to snorting and smoking black tar. Since a user cannot know the amount of intake of "cut" drugs, overdosing can occur. Tolerance of the drug, physical dependence, and psychological need are elements of addiction. Overcoming the psychological need for the drug is the most difficult obstacle to face with relapse inevitable if the deeper issue of addiction are not resolved. To examine a user for injection sites, an officer must see certain symptoms present among them marks from injections, contraction of the pupils, and slowed muscle reaction are symptoms of heroin use.