The bariatric surgery is widely used to treat obesity. Roux-en-Y gastric bypass is one of the most frequently surgical methods performed and combines restrictive and malabsorptive procedures. Different data suggest that this surgery may modify drug absorption and we think it would be clinically relevant to describe the consequences of gastric bypass on drug systemic exposure in obese patients, since no data on the comparison between the pharmacokinetics (PK) of a drug before and after surgery are available and help to predict the drugs posology.The investigators decided to study the morphine because there is a lack of information about the PK, pharmacodynamics (PD) et pharmacogenetics (PG) of morphine in obese subjects, in contrary with anaesthetic drugs. This is a drug with a narrow therapeutic range frequently prescribed in obese patients.

Inclusion Criteria:

• Patients aged between 18 and 60 years old.
• Patients with morbid obesity (IMC > 40 kg/m²) or severe obesity (IMC=35-40 kg/m²) with co morbidities (sleep apnea syndrome or hypertension or steatosis hepatitis) with a favourable decision of a multidisciplinary team for a gastric bypass.
• Patient agreeing to go 3 times for a one day hospitalisation in the URT of the Lariboisière Hospital for the morphine PK/PD.
• Patient with a previous medical examination.
• Patient giving its well-informed and free consent after information.

Exclusion Criteria:

• diabetes
• concomitant sedative, antidepressive or analgesic treatments or drugs unadvised with morphine
• untreated sleep apnea syndrom, hypoxia (PaO2<70mmHg) or hypercapnia (PaCO2>45mmHg), anaemia <10g/dL, ASAT or ALAT>3N