Psychological self-medication in mammals: A dialogue between natural observations and laboratory research

The use of plants with medicinal properties by nonhuman animals has been extensively documented. However, little emphasis has been placed on evidence suggesting that animals also appear to use/consume substances with psychoactive properties. This psychological self-medication (PsycSM) hypothesis posits that the voluntary use of substances that alter emotional/mental states is guided by the substance's ability to reduce negative states or promote positive states. The objective of this narrative review is to provide a rigorous interpretation and critique of findings, point out contributions and limitations of available research, and provide a synthesis useful to design further studies aimed at testing the PsycSM hypothesis. Criteria derived from this hypothesis are used to establish behaviors as PsycSM examples. Areas for potentially useful interactions between field and laboratory research are specified. Anecdotal evidence from field observations suggests that animals voluntarily consume substances that affect their emotional state. Although laboratory research reviewed herein suggests that nonhuman animals intentionally consume substances that can affect their emotional/mental state, the underlying mechanisms are still poorly understood. One such mechanism is hypothesized to involve a reinforcement process produced by a change in the internal state contingent on substance consumption, whether to reduce negative emotions or to enhance positive emotions. The criteria for PsycSM developed in this review may guide further research into these effects. (PsycInfo Database Record (c) 2026 APA, all rights reserved).