Research published in the Journal of Clinical Psychiatry found no apparent relationship between β-endorphin level and self-injurious behavior or pain reactivity in individuals with autism. These findings undermine the opioid theory of autism, which suggests that patients with autism may have hyporeactivity to pain as a result of excessive brain opioid activity.

Investigators assessed behavioral pain reactivity and self-injurious behaviors through observational situations for 74 children and adolescents with autism and intellectual disability. Plasma β-endorphin concentrations were measured in 57 participants using 2 methods: an immunoradiometric sandwich assay and a radioimmunoassay procedure. Per parental and caregiver observation, respectively, 50.0% and 70.3% of individuals with autism displayed self-injurious behavior, most frequently head banging and hand biting. 

An absence or decrease of overall behavioral pain reactivity was observed in 68.6% and 34.2% of patients, according to parental and caregiver observation, respectively. According to parental evaluation, individuals with hyporeactivity to accidental pain stimuli displayed higher rates of self-biting (P <.01). No significant relationships were found between plasma β-endorphin levels and self-injurious behavior levels or pain reactivity, by either immunoassay method.

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These results challenge modern theories that disturbances in the endogenous opioid of patients with autism may affect pain reactivity. The large number of study subjects, the use of 2 different immunoassay methods, and the highly quantitative ratings system used strengthen study results, said researchers.

Investigators suggested that future studies explore the neuroendocrine aspects of autism and noted that these data do not thoroughly dismiss a potential role of the opioid system in the pathophysiology of autism.


Tordjman S, Anderson GM, Charrier A, et al. Relationships between self-injurious behaviors, pain reactivity, and β-endorphin in children and adolescents with autismJ Clin Psychiatry. 2018;79(2):16m10089.