Breakthrough Pain: Acute pain that comes on rapidly despite the use of pain medication.Administration, Buccal: Administration of a soluble dosage form between the cheek and gingiva. It may involve direct application of a drug onto the buccal mucosa, as by painting or spraying.Fentanyl: A potent narcotic analgesic, abuse of which leads to habituation or addiction. It is primarily a mu-opioid agonist. Fentanyl is also used as an adjunct to general anesthetics, and as an anesthetic for induction and maintenance. (From Martindale, The Extra Pharmacopoeia, 30th ed, p1078)Labor Pain: Pain associated with OBSTETRIC LABOR in CHILDBIRTH. It is caused primarily by UTERINE CONTRACTION as well as pressure on the CERVIX; BLADDER; and the GASTROINTESTINAL TRACT. Labor pain mostly occurs in the ABDOMEN; the GROIN; and the BACK.Analgesics, Opioid: Compounds with activity like OPIATE ALKALOIDS, acting at OPIOID RECEPTORS. Properties include induction of ANALGESIA or NARCOSIS.Pain: An unpleasant sensation induced by noxious stimuli which are detected by NERVE ENDINGS of NOCICEPTIVE NEURONS.Analgesia, Epidural: The relief of pain without loss of consciousness through the introduction of an analgesic agent into the epidural space of the vertebral canal. It is differentiated from ANESTHESIA, EPIDURAL which refers to the state of insensitivity to sensation.Pain Measurement: Scales, questionnaires, tests, and other methods used to assess pain severity and duration in patients or experimental animals to aid in diagnosis, therapy, and physiological studies.Double-Blind Method: A method of studying a drug or procedure in which both the subjects and investigators are kept unaware of who is actually getting which specific treatment.Neoplasms: New abnormal growth of tissue. Malignant neoplasms show a greater degree of anaplasia and have the properties of invasion and metastasis, compared to benign neoplasms.Chronic Disease: Diseases which have one or more of the following characteristics: they are permanent, leave residual disability, are caused by nonreversible pathological alteration, require special training of the patient for rehabilitation, or may be expected to require a long period of supervision, observation, or care. (Dictionary of Health Services Management, 2d ed)Chronic Pain: Aching sensation that persists for more than a few months. It may or may not be associated with trauma or disease, and may persist after the initial injury has healed. Its localization, character, and timing are more vague than with acute pain.Pain Management: A form of therapy that employs a coordinated and interdisciplinary approach for easing the suffering and improving the quality of life of those experiencing pain.Pain Threshold: Amount of stimulation required before the sensation of pain is experienced.Pain, Postoperative: Pain during the period after surgery.Radionuclide Generators: Separation systems containing a relatively long-lived parent radionuclide which produces a short-lived daughter in its decay scheme. The daughter can be periodically extracted (milked) by means of an appropriate eluting agent.Low Back Pain: Acute or chronic pain in the lumbar or sacral regions, which may be associated with musculo-ligamentous SPRAINS AND STRAINS; INTERVERTEBRAL DISK DISPLACEMENT; and other conditions.Back Pain: Acute or chronic pain located in the posterior regions of the THORAX; LUMBOSACRAL REGION; or the adjacent regions.Pain, Intractable: Persistent pain that is refractory to some or all forms of treatment.Abdominal Pain: Sensation of discomfort, distress, or agony in the abdominal region.Neck Pain: Discomfort or more intense forms of pain that are localized to the cervical region. This term generally refers to pain in the posterior or lateral regions of the neck.Pelvic Pain: Pain in the pelvic region of genital and non-genital origin and of organic or psychogenic etiology. Frequent causes of pain are distension or contraction of hollow viscera, rapid stretching of the capsule of a solid organ, chemical irritation, tissue ischemia, and neuritis secondary to inflammatory, neoplastic, or fibrotic processes in adjacent organs. (Kase, Weingold & Gershenson: Principles and Practice of Clinical Gynecology, 2d ed, pp479-508)Pain Perception: The process by which PAIN is recognized and interpreted by the brain.Facial Pain: Pain in the facial region including orofacial pain and craniofacial pain. Associated conditions include local inflammatory and neoplastic disorders and neuralgic syndromes involving the trigeminal, facial, and glossopharyngeal nerves. Conditions which feature recurrent or persistent facial pain as the primary manifestation of disease are referred to as FACIAL PAIN SYNDROMES.