N-methyl-D-aspartate (NMDA) receptors are involved in the development of neuropathic pain. Ketamine, a non-competitive NMDA receptor antagonist, has in several case reports given pain relief but efficacy in dosages tolerated in long-term ketamine treatment is unknown. Another substance with an antagonist action at NMDA receptors and which is approved for peroral administration is dextromethorphan. In a randomized study dextromethorphan was no better than placebo for neuropathic pain but this does not exclude efficacy in selected patients. We report a patient with severe post-herpetic pain resistant to conventional pain treatment which was treated with ketamine for 4 years with good pain relief. The practical application of long-term treatment in different administration forms of ketamine is described. The patient also responded with pain relief in a double-blind trial with oral dextromethorphan.