Most drugs that act at the synaptic gap will lead to tolerance and a possible "rebound" effect, which can actually cause an augmentation of the original symptoms they were supposed to treat. Benzodiazepines are particularly well-know for this. Benzos have been known to cause a terrible withdrawal syndrome, with some percentage of users actually developing a "post-withdrawal" syndrome that lasts years or perhaps permanently: strange neurological symptoms, severe tinnitus, muscle spasms, anxiety a hundred times worse than before the drugs, etc. This syndrome can resemble multiple sclerosis or a myriad of other grave neurological disorders. In some people its worse than others, nobody knows why. SSRIs are no walk in the park either. Use at your own risk.