This study is being conducted to evaluate how the body absorbs and processes the immediate release (IR) and sustained release (XR) medication venlafaxine (Effexor®). Subject who are 1-3 years post gastric bypass or sleeve gastrectomy surgery will be invited to participate. Non-surgical controls will also be enrolled based on a matching criteria to post gastric bypass subjects. Participants will be asked to complete two 12-hour study days approximately 11 days apart. This study will enroll up to 30 participants.

Inclusion Criteria:

1. Male or Female
2. Age 18-65 (inclusive, at time of informed consent)
3. No tobacco use in the past three months.
4. Underwent Roux-en-Y Gastric Bypass or Sleeve Gastrectomy weight loss surgery 12-36 months prior to study OR has not had a weight loss surgery but matches the gastric bypass patients on age, gender, and BMI. Criteria for matching will be established at the beginning of the study in consultation with the statistical team and addressed as needed with the input of the biostatistician.
5. Ability to read, write and understand English

Exclusion Criteria:

1. Taking a medication that has a clinically significant interaction with venlafaxine or an interaction that may alter the study data.
2. Hypersensitivity to venlafaxine or any excipient contained within the dosage forms
3. Inability to tolerate repeated blood draws.
4. Any history of bipoloar disorder or a psychotic disorder.
5. Current major depressive disorder or current suicidality.
6. Alcohol or substance dependence in the past year.
7. Currently pregnant or lactating or unwillingness to use medically accepted contraception during study
8. Taking a medication which significantly alters gastrointesinal transit time or significantly reduces acid secretion (e.g. routine use of proton pump inhibitors, H2 antagonists, sucralfate).
9. Medical conditon which may increase participant risk with venlafaxine or uncontrolled hypertension at the discretion of the examining provider.
10. Hepatic insufficiency as defined by any hepatic enzyme greater than 3x the upper limit of normal or other hepatic laboratory abnormalities at the discretion of the medical provider.
11. Renal impairment as evidence by a calculated creatinine clearance of less than or equal to 70ml/min or other abnormality on a renal panel that the medical provider feels puts the participant at risk.
12. Poor or ultra-rapid Cytochrome P450 2D6 metabolizer.
13. Self reported history of viral hepatits or HIV.
14. Positive urine drug screen unless documented prescription of a non-interacting medication.