Tramadol is an opioid analgesic, which is widely used in the treatment of acute and neuropathic pain. Treatment of neuropathic pain often requires a combination of pain medications due to the complex nature of neuropathic pain and frequent inadequate response to drug treatment. Common drugs used concomitantly with tramadol are SNRI antidepressant duloxetine and anticonvulsants such as pregabalin. Both tramadol and duloxetine have serotonergic effects and duloxetine has also a potential to inhibit metabolism of tramadol. The objective of the study is to investigate the pharmacokinetics and pharmacodynamic interaction of oral tramadol with duloxetine and pregabalin in patients with chronic neuropathic pain due to postherpetic neuralgia or diabetic polyneuropathy. All subjects will receive tramadol and duloxetine or tramadol and pregabalin in a randomized double-blind order. Primary end point is O-desmethyltramadol concentration.