This document, Chronic Amphetamine Use and Abuse, goes into great detail on the effects of chronic amphetamine use and abuse, and will hopefully help to answer your question:
Neurotoxic Effects of Stimulant Drugs
"Sustained high-dose administration of amphetamines (especially methamphetamine) to experimental animals produces a persistent depletion of [dopamine] which is associated with terminal degeneration (62, 182, 195), as well as neuronal chromatolysis in the brain stem, cortex and striatum (42, 182). ..."
I expect that different drugs will have different effects, both long-term and short-term, and since you didn't specify a particular drug I thought amphetamines would be a good one to look at because crystal meth in particular is known to be one of the dangerous ones (particularly with illegal meth labs, that police keep having to shut down, and then reports on the radio follow with observations that there may have been attempts to save money or create something unique by substituting or mixing in other chemicals which, if true, could result in the creation of an even more dangerous substance).