Sixty participants in normal health, 18 to 50 years of age, who meet the entry criteria, will be randomized to one of the three treatments in treatment segment 1. Each arm will ingest three consecutive 20 mg daily doses of paroxetine. Twelve hours after the first paroxetine dose, subjects will be randomized to one of the treatment sequences to ingest a total of 9 doses of Omnitram, tramadol, or placebo (one dose every 6 hours). Immediately before the 9th dose a blood sample will be collected to quantify plasma Omnitram, tramadol, and paroxetine. After the 9th study drug dose, six blood samples will be collected (1.0, 1.5, 2.0, 2.5, 4.0, and 8.0 hours after the 9th dose is administered) to quantify the Omnitram and tramadol. After the 9th dose, pain tolerance will be assessed with a cold pressor test (immersion of hand in ice cold water). Participants will washout for 11-15 days after treatment 1 and treatment 2. The study will analyze treatment side effects, the pharmacokinetics, and pain tolerance.