The opioid induced neurotoxicity (OIN) is an adverse effect for opioids use, described in the last years. Because the accumulation of toxic metabolites, especially M3 Glucuronide of morphine, cause neuronal hiperexcitability, patients can develop cognitive failure, delirium, hallucinations, myoclonus, seizures and hyperalgesia. The most vulnerable patients are old people, patients with dehydration and renal failure. Its treatment include prevention, with the management of trigger factors , decrease or change opioids and symptomatic management, trying to keep the good control of pain.