PRL and Respiratory insufficiency due to muscle weakness: The activation of the aforementioned receptors produces supraspinal and spinal analgesia, increased prolactin release (μ1), respiratory depression (μ2), decreased gastrointestinal motility (μ2), sedation, miosis, euphoria (by binding of endorphins), drug dependence (by triggering euphoria and stimulus) immunosuppression (μ3), and hyperalgesia (μ1 and μ2) [3,24,51,52,53].