ingesting psilocybin more than about once a week can lead to diminished effects. Tolerance dissipates after a few days, so doses can be spaced several days apart to avoid the effect. A cross-tolerance can develop between psilocybin and the pharmacologically similar LSD, and between psilocybin and phenethylamines such as mescaline and DOM. Repeated use of psilocybin does not lead to physical dependence. A 2008 study concluded that, based on US data from the period 2000–2002, adolescent-onset (defined here as ages 11–17) usage of hallucinogenic drugs (including psilocybin) did not increase the risk of drug dependence in adulthood; this was in contrast to adolescent usage of cannabis, cocaine, inhalants, anxiolytic medicines, and stimulants, all of which were associated with "an excess risk of developing clinical features associated with drug dependence". Likewise, a 2010 Dutch study ranked the relative harm of psilocybin mushrooms compared to a selection of 19 recreational drugs, including alcohol, cannabis, cocaine, ecstasy, heroin, and tobacco. Psilocybin mushrooms were ranked as the illicit drug with the lowest harm, corroborating conclusions reached earlier by expert groups in the United Kingdom. Pharmacology Pharmacodynamics Psilocybin is a psychoplastogen, which refers to a compound capable of promoting rapid and sustained neuroplasticity. Psilocybin is rapidly dephosphorylated in the body to psilocin, which is an agonist for several serotonin receptors, which are also known as 5-hydroxytryptamine (5-HT) receptors. In rats, psilocin binds with high affinity to 5-HT2A receptors and low affinity to 5-HT1 receptors, including 5-HT1A and 5-HT1D; effects are also mediated via 5-HT2C receptors. The psychotomimetic (psychosis-mimicking) effects of psilocin can be blocked in a dose-dependent fashion by the 5-HT2A antagonist drug ketanserin. Various lines of evidence have shown that interactions with non-5-HT2 receptors also contribute to the subjective and behavioral effects of the drug. For example, psilocin indirectly increases the concentration of the neurotransmitter dopamine in the basal ganglia, and some psychotomimetic symptoms of psilocin are reduced by haloperidol, a non-selective dopamine receptor antagonist. Taken together, these suggest that there may be an indirect dopaminergic contribution to psilocin's psychotomimetic effects. Psilocybin and psilocin have no affinity for dopamine receptor D2, unlike another common 5-HT receptor agonist, LSD. Psilocin antagonizes H1 receptors with moderate affinity, compared to LSD which has a lower affinity. Serotonin receptors are located in numerous parts of the brain, including the cerebral cortex, and are involved in a wide range of functions, including regulation of mood, motivation, body temperature, appetite