The purpose of this study is to compare the number of osteoarthritis patients receiving titrated or increasing doses of tramadol hydrochloride/acetaminophen combination tablet (titration group) who discontinue treatment due to adverse events to the number of patients receiving a non-titrated dose of tramadol/hydrochloride/acetaminophen combination tablet who discontinue treatment due to adverse events.

This is a randomized (study drug assigned by chance), multicenter, double-blind (neither physician nor patient knows the name of the assigned drug), add-on study (NSAIDs or COX-2 medications maintained at pre-study dose.) to investigate the tolerability of titrated doses of tramadol hydrochloride/acetaminophen combination tablet in osteoarthritis patients taking NSAIDS or COX-2 inhibitors (both are drugs used to treat inflammation and pain) compared to non-titrated doses of tramadol hydrochloride/acetaminophen combination tablet. The patients will be randomized into 2 treatment groups. First group (the titration group) receives 1 tablet of tramadol hydrochloride/acetaminophen in the morning, 1 tablet of placebo in the afternoon and evening for 3 days, then 1 tablet of tramadol hydrochloride/acetaminophen in the morning, 1 tablet of placebo in the afternoon and 1 tablet of tramadol hydrochloride/acetaminophen in the evening for 4 days, and the 1 tablet of tramadol hydrochloride/acetaminophen three times daily for next 7 days. The second group (non-titration group) will receive 1 tablet of tramadol hydrochloride/acetaminophen three times daily without titration for 14 days. The study hypothesis is the first group's discontinuation rate due to adverse events is lower than that of second group. Safety assessments include adverse event, vital sign and clinical lab monitoring. First group will receive 1 tablet of tramadol hydrochloride/acetaminophen and 2 tablets of placebo for 3 days, 2 tablets of tramadol hydrochloride/acetaminophen and 1 tablet of placebo for 4 days, and 3 tablets of tramadol hydrochloride/acetaminophen for 7 days. Second group will receive 3 tablets of tramadol hydrochloride/acetaminophen for 14 days.

Inclusion Criteria:

• Must have osteoarthritis of the knee for at least one year based on clinical and radiological criteria
• Must have been on a stable daily dose of a NSAID for at least two weeks prior to randomization
• Must have a pain intensity Numeric Rating Scale (NRS) score 4 or more as a average intensity for 48 hours. (On a Numeric Rating Scale of 0 to 10, with 0 being no pain at all and 10 being the worst pain imaginable)

Exclusion Criteria:

• Have taken tramadol HCL within 14 days prior to randomization
• Have taken acetaminophen within 7 days prior to randomization
• Have received other pain medication (including topical medication and/or anesthetics), sedative hypnotics, or muscle relaxants within a period of less than five half-lives of the given medication prior to randomization