To evaluate the effects of multiple oral doses of desvenlafaxine sustained release (DVS SR) and paroxetine on the biotransformation of codeine to morphine in healthy subjects. To assess the safety and tolerability of DVS SR and paroxetine when coadministered with codeine to healthy subjects.

This is a randomized, open-label, inpatient/outpatient, 3-period crossover study in healthy subjects.The study will consist of 3 treatment periods. In treatment period 1, subjects will be randomly assigned on study day 1. A single 60-mg dose of codeine will be administered to all subjects. In periods 2 and 3, subjects will receive either DVS SR 100 mg/day or paroxetine 20 mg/day until the steady state is reached. At steady state, subjects will receive codeine 60 mg concomitantly with either DVS SR 100 mg or paroxetine 20 mg, depending on the treatment sequence to which they are assigned. DVS SR 100 mg or paroxetine 20 mg administration will continue for an additional 2 days. In treatment period 3, subjects will receive the alternative treatment sequence.

Inclusion criteria:

• Healthy men and nonlactating and nonpregnant women of nonchildbearing potential aged from 18 to 45 years.
• Body mass index in the range of 18 to 30 kg/m2 and body weight ≥50 kg.
• Nonsmoker or smoker of fewer than 10 cigarettes per day as determined by history.

Exclusion criteria:

• History of bronchial asthma, of seizure disorder (other than a single childhood febrile seizure)and of any clinically important drug allergy and any known allergy to desvenlafaxine, venlafaxine, paroxetine, codeine, or any of the non-active excipients in the dosage forms.
• Use of any CYP2D6 inhibitors or inducers within 30 days before test article administration.
• Demonstration of a positive orthostatic test at screening.