Amphetamine psychosis has been induced in experimental studies of amphetamine abusers by doses smaller than those that were originally involved in its gradual development (16, 186). In case studies, Kramer (126), Ellinwood (60) and Ellinwood and Petrie (49) demonstrated that, once an individual has experienced amphetamine paranoia, it readily reappears at lower doses or earlier in a drug run. In the 1991 study by Satel et al. (184), three-fourths of the stimulant abusers who became paranoid said these experiences clearly worsened with continued drug use, and most described a more rapid onset. Similar results were reported by Brady et al. (23). Bartlett and colleagues (15) also described increasing suspiciousness and paranoia but not increased euphoria with repeated cocaine use. In addition to the psychosis being induced by lower doses over time, even stress has been described as capable of inducing psychosis in some patients (186, 214).
The biological substrates of these progressive conditions have most often been ascribed to tolerance and/or sensitization. Tolerance is most strongly reflected by the fact that both lower animals and humans can sustain even lethal doses of amphetamine after chronic administration. Sensitization reflects the phenomenon that even though a high dose may have been necessary to induce end-stage behavioral pathology, moderate doses given over a shorter duration are able to activate the behavioral pathology. Both tolerance and sensitization are ‘black box' phenomena for which the initiating, maintaining, and reactivation mechanisms are only partially understood and at times are controversial in the literature. Even the concept of sensitization vs. tolerance and their contribution to the end-stage behaviors continue to be debated. Because sensitization can be induced by much lower doses of stimulants, some have argued that the initiation of stimulant abuse patterns is based more on an early stage of sensitization-like augmentation of conditioned behaviors, whereas the chronic evolution of psychosis more often also requires tolerance-inducing sustained high doses of amphetamine. (74, 134). Lieberman et al. (134) make the case for a tolerance/neurotoxic effects that would explain both the psychosis as well as the withdrawal burnout condition of former ‘speed freaks' (10, Utena et al., 1975).
Sensitization is a robust, extremely long-lasting effect of repeated, especially intermittent, stimulant administration. The rapidity of the establishment of sensitization is influenced by many factors, including dosing parameters, negative influence of male gonadal hormones, and the time after the last dose of amphetamine (177). With intermittent stimulant administration, there is progressive enhancement of locomotor hyperactivity and/or stereotyped behavior (194, 205). In contrast, continuous administration of amphetamine or cocaine induces a tolerance to subsequent injections for at least a week after chronic dosing, while intermittent dosing induces a reverse tolerance (30, 115, 223). Yet, high-dose (10–30 mg/kg/day), continuous administration or multi-day dosing eventually induces bizarre hyper-reactive and fragmented behavior, including fearful responsivity, which is more easily reactivated after withdrawal in a manner similar to other sensitization responses to stimulants.
http://www.acnp.org/G4/GN401000166/CH162.htm
Sensitization and tolerance are definitely more complex and less understood than most other drug-related subjects. One important point is that humans can develop tolerance to certain effects of speed (such as euphoria, anorexia, etc) while simultaneously experiencing sensitization to other effects (stereotyped, ritualistic behavior). Over time, tolerance requires that you take larger doses to get a buzz, but sensitization only needs doses smaller than what was originally used to produce effects such as stereotypies. So it is no wonder that long-term speed users often become unable to tolerate the drug effects anymore (even a small dose with minimal euphoria could produce some of the psychopathological effects that normally only occur with high doses). Still, I don't want to oversimplify the complex nature of the subject. Visit the above link if you have any questions about experiments and you can also watch some videos of laboratory animals and how they behave after being on an IV of meth for a solid week.