The aim of this trial is to investigate the effectiveness (level of pain control) and safety (side effects) of Tapentadol Prolonged Release (ER) compared to a tablet with no active ingredient drug (placebo) and a corresponding dose of Morphine (an opioid commonly used to treat tumor related pain). This trial is a randomized, double-blind (neither investigator nor patient will know which treatment was received), active- and placebo-controlled, parallel-group, randomized-withdrawal, multicenter trial. To maintain the blind all subjects were re-randomized at the start of the maintenance period. To maintain the blind all tapentadol subjects were re-randomized at the start of the maintenance period. Subjects that received morphine in the titration period continued in the maintenance period on morphine.