a half times that of caffeine. When administered intravenously in rabbits, psilocybin's LD50 is approximately 12.5 mg/kg. Psilocybin comprises approximately 1% of the weight of Psilocybe cubensis mushrooms, and so nearly of dried mushrooms, or of fresh mushrooms, would be required for a person to reach the 280 mg/kg LD50 value of rats. Based on the results of animal studies, the lethal dose of psilocybin has been extrapolated to be 6 grams, 1000 times greater than the effective dose of 6 milligrams. The Registry of Toxic Effects of Chemical Substances assigns psilocybin a relatively high therapeutic index of 641 (higher values correspond to a better safety profile); for comparison, the therapeutic indices of aspirin and nicotine are 199 and 21, respectively. The lethal dose from psilocybin toxicity alone is unknown at recreational or medicinal levels, and has rarely been documented—, only two cases attributed to overdosing on hallucinogenic mushrooms (without concurrent use of other drugs) have been reported in the scientific literature and may involve other factors aside from psilocybin. Psychiatric Panic reactions can occur after consumption of psilocybin-containing mushrooms, especially if the ingestion is accidental or otherwise unexpected. Reactions characterized by violent behavior, suicidal thoughts, schizophrenia-like psychosis, and convulsions have been reported in the literature. A 2005 survey conducted in the United Kingdom found that almost a quarter of those who had used psilocybin mushrooms in the past year had experienced a panic attack. Other adverse effects less frequently reported include paranoia, confusion, prolonged derealization (disconnection from reality), and mania. Psilocybin usage can temporarily induce a state of depersonalization disorder. Usage by those with schizophrenia can induce acute psychotic states requiring hospitalization. The similarity of psilocybin-induced symptoms to those of schizophrenia has made the drug a useful research tool in behavioral and neuroimaging studies of this psychotic disorder. In both cases, psychotic symptoms are thought to arise from a "deficient gating of sensory and cognitive information" in the brain that ultimately lead to "cognitive fragmentation and psychosis". Flashbacks (spontaneous recurrences of a previous psilocybin experience) can occur long after having used psilocybin mushrooms. Hallucinogen persisting perception disorder (HPPD) is characterized by a continual presence of visual disturbances similar to those generated by psychedelic substances. Neither flashbacks nor HPPD are commonly associated with psilocybin usage, and correlations between HPPD and psychedelics are further obscured by polydrug use and other variables. Tolerance and dependence Tolerance to psilocybin builds and dissipates quickly;