This study will establish the dose equivalence and the safety and effectiveness of the Immediate Release (IR) and Extended Release (ER) forms of tapentadol (CG5503) to support the conversion from IR to ER, and ER to IR use. Dose equivalence will be examined in patients diagnosed with moderate-to-severe, chronic Low Back Pain (LBP) requiring drug treatment for at least 3 months, and who are dissatisfied with current therapy. The study consists of 5 periods: a screening period during which patients are evaluated for study eligibility; a 21-day open-label period to find the best, stable dose of tapentadol (CG5503) IR for each patient individually; a 14-day double-blind period when patients are randomly chosen either to continue for 14 days on the stable IR dose from the open-label period or switch to the ER form; a second, 14-day period during which patients switch to whichever form of tapentadol (CG5503) they did not take during the first 14-day period (the total daily dose [TDD] remains approximately equivalent for the IR and ER forms throughout both double-blind periods); and a follow-up period. During the study, pain levels will be recorded and overall safety measures taken. The expectation (thought) is that approximately equivalent doses of both forms of tapentadol (CG5503) provide equivalent effectiveness and safety and that the two forms can be directly converted by dividing the total daily dose by the number of times the drug is taken each day. During the 21-day open-label period, 50, 75 or 100mg of the IR form is given orally every 4 or 6 hours, starting with 50mg every 6 hours. Then, the dose, the frequency of giving the drug, or both may be increased, to a maximum TDD of 500mg, or decreased in 50 mg increments, with minimum TDD of 200 mg, until the optimal stable dose for a patient is found. During the 2 double-blind periods, a TDD approximately equivalent to the stable open-label dose is given orally in IR (or ER) form or placebo.