The purpose of the study is to determine whether a significant and predictable change in bioavailability of extended-release venlafaxine occurs following Roux-en-Y gastric bypass.

Inclusion criteria:

- Approved to undergo Roux-en-Y gastric bypass surgery (cost of surgery is NOT included in the study)

Exclusion criteria:

• Pregnant
• Allergy to venlafaxine or desvenlafaxine
• Psychiatric hospitalization within the prior 12 months
• Active professional treatment for recent substance abuse within 12 months of abstinence
• Ongoing psychologic issues, such as personality disorders, difficulties as a trauma survivor, or difficulties with depression unless a stable, documented course of treatment by a licensed mental health professional is available
• Current use of any of the following medications/supplements: 5-hydroxytryptophan, almotriptan, amitriptyline, amoxapine, amoxicillin-clavulanate, amphetamine-dextroamphetamine, atazanavir, bupropion, cinacalcet, citalopram, clarithromycin, clomipramine, desipramine, desvenlafaxine, dextroamphetamine, dextromethorphan, doxepin, duloxetine, eletriptan, entacapone, escitalopram, fluoxetine, fluvoxamine, frovatriptan, haloperidol, imipramine, isocarboxazid, itraconazole, jujube seed extract, linezolid, maprotiline, methylene blue, metoclopramide, metoprolol, milnacipran, mirtazapine, naratriptan, nefazodone, nelfinavir, nortriptyline, paroxetine, phenelzine, procarbazine, protriptyline, quinidine, rasagiline, ritonavir, rizatriptan, saquinavir, selegiline, sertraline, St. John's wort, sumatriptan, tapentadol, terbinafine, toremifene, tramadol, tranylcypromine, trazodone, trifluoperazine, trimipramine, tryptophan, L-tryptophan, venlafaxine, vilazodone, zolmitriptan