The neurobiology of synaesthesia is receiving growing attention in the search for insights into consciousness, such as the binding problem. One way of decoding the neurocognitive mechanisms underlying this phenomenon is to investigate the induction of synaesthesia via neurochemical agents, as commonly occurs with psychedelic substances. How synaesthesia is affected by drugs can also help inform us of the neural mechanisms underlying this condition. To address these questions we surveyed a sample of recreational drug users regarding the prevalence, type and frequency of synaesthesia under the influence of psychedelics and other psychoactive substances. The results indicate that synaesthesia is frequently experienced following the consumption of serotonergic agonists such as LSD and psilocybin and that these same drugs appear to augment synaesthesia in congenital synaesthetes. These results implicate the serotonergic system in the experience of synaesthesia.