Identification of the genetic polymorphisms that could be correlated either with a better clinical response or with a major predisposition of patients to develop tolerance and/or side effects to the treatment with morphine.

Valuation of the rescue doses necessary to maintain NRS<4 in the first 24 hours post-surgery in the two groups of patients, A e B. Group A: homozygous patients for the more frequent allele of the polymorphism A118G of OPRM1 gene (about 80%); group B: both homozygous and heterozygous patients for the less frequent allele (about 20%).

Inclusion criteria:

• Males and females over 18 years, under 75 years, scheduled for postoperative pain control with continuous morphine administration
• HIV negative
• Classification American Society of Anesthesiologists (ASA) I: without systemic disease
• Classification ASA II or III (mild systemic disease or severe systemic disease that limits the activity without invalidity).
• Undergoing abdominal and urologic major surgery (neither urgent nor emergency surgery)
• Signed informed consent

Exclusion criteria:

• Usual assumption of analgesic opioids
• Cognitive alterations nor mental retardation
• Severe hepatic/renal insufficiency (cholinesterase <3000 mU/ml, total bilirubinaemia <2 mg/dl and creatininaemia <1.2 mg/dl)
• Inpatients in intensive therapy, either with sedation and/or mechanic ventilation.
• Allergies to morphine and derivates

Pazients scheduled for major abdominal or urological surgery with postoperative pain control with continuous morphine administration