Classical psychedelic substances are serotonergic agonists that act essentially on the 5HT2A neurotransmission system. These substances can promote altered states of consciousness, as well as visual, affective, mood, and cognitive changes. Historically, they have been used for ritualistic, recreational, and medicinal purposes. Studies indicate that these substances have low toxicity, low risk of dependence and overdose. There is current scientific interest in the use of these substances as a treatment for a variety of conditions, and while many studies seek to objectively investigate outcomes linked to their use, the subjective contents of the experience of users is seldom explored. Since a first-person perspective is central to the psychedelic experience, this thesis sought to investigate subjective experiences linked to psychedelics through a variety of approaches. First, we sought to adapt and validate the Ego Dissolution Inventory (EDI), an instrument widely used by researchers to assess a central feature of the psychedelic experience, to the Brazilian context (Article 1). Then, we sought to explore public negative (Article 2) and positive (Article 3) reports of psilocybin (the active principle of magic mushrooms) users, through the Reinert method, which quantitatively analyses transcribed speeches. Finally, the possible influence of altered states of consciousness (ASC), including those linked to use of psychoactive substances, in human evolution was discussed (Article 4), suggesting future research to explore this hypothesis.