The purpose of this study is to evaluate the efficacy, safety, and tolerability of orally administered tapentadol extended release (ER) at dosages of 100 to 250 mg twice daily compared with placebo in patients with moderate to severe pain due to chronic, painful diabetic peripheral neuropathy (DPN) who tolerated tapentadol (ER) and have an initial treatment effect (pain improvement) after a 3-week, open-label titration period.