Relative percentages showed a mean 24.1% higher opiorphin concentration in chronic conditions (Burning Mouth Syndrome, Oral Potentially Malignant Diseases and Temporomandibular Disorder) compared to controls; 33.2% higher opiorphin in sustained pain (Symptomatic Irreversible Pulpitis, Symptomatic Apical Periodontitis, Painful Oral Soft-tissue conditions); and 21.7% higher opiorphin after stimuli (Corneal Foreign Body, Capsaicin). The gene discussed is OPRPN; the disease is periapical periodontitis.