A review of published studies finds consistent evidence that serotonin agonists can temporarily produce synaesthesia, a condition where senses blend, such as seeing sounds. However, the research has many methodological flaws and little experimental work has been done. While the findings point to the serotonergic system's role in synaesthesia, the limitations prevent firm conclusions about whether chemicals truly induce genuine synaesthesia.
Lysergic acid diethylamide (LSD) induces synaesthesia-like experiences in non-synaesthetes, but these experiences do not meet two key criteria for genuine synaesthesia: consistency of stimulus-colour associations over time and inducer specificity. In a placebo-controlled pilot study, participants reported more spontaneous synaesthesia-like experiences under LSD compared to placebo, yet their colour responses to standardized graphemes and sounds did not differ across conditions in consistency or specificity. Individual differences in absorption propensity were linked to some effects. The findings suggest that LSD-induced synaesthesia-like experiences are not equivalent to genuine synaesthesia.