Heroin was originally developed as a pain medication but was found to be highly addictive. It is classified as a depressant that slows brain function and can be administered via injection, snorting, smoking, or pills. Long term heroin use can damage teeth and gums, cause constipation and cold sweats, and weaken the immune system. Mixing heroin with alcohol is especially dangerous since both are depressants that can lead to overdose.