A recent study has shown that a single use of cocaine appears to change the neuroanatomy of mice brains. I have had it frequently reported to me that the first experience of cocaine makes one want another cocaine-induced experience, but also that the subsequent experiences never match the initial experience. That the first experience changes the structure of neurons suggests this might be true.
This study was done on mice, which presumably may be more easily affected by the single dose than larger humans, but how much different (if at all)? The brain is remarkably plastic and, as the researchers point out, the ease with which cocaine changes the brain likewise suggest the opportunity to re-change the brain as well. However, there are no chemicals that will un-change the brain as effectively or quickly as chemicals can change it.