The aim of this study is to evaluate if memantine or dextromethorphan gave in relay to ketamine maintains or induces a decrease of pain intensity.

In clinical, after ketamine cure, clinicians have often difficulties to treat patients in order to maintain analgesia or suggest another treatment if analgesia induced by ketamine was not effective, which occurs in one quarter of patients. It will be very useful for monitoring of painful patients evaluate if memantine or dextromethorphan could be an effective therapeutic alternative in neuropathic pain treatment.

The aim of this study is to evaluate if memantine or dextromethorphan gave in relay to ketamine maintains or induces a decrease of pain intensity.

In clinical, after ketamine cure, clinicians have often difficulties to treat patients in order to maintain analgesia or suggest another treatment if analgesia induced by ketamine was not effective, which occurs in one quarter of patients. It will be very useful for monitoring of painful patients evaluate if memantine or dextromethorphan could be an effective therapeutic alternative in neuropathic pain treatment.

Inclusion Criteria:

• > 18 years old
• patient suffering chronic neuropathic pain
• All chronic pain is retained except central or diabetic pain
• Answering patient at ketamine in pain treatment by investigator, and having already received ketamine
• Patient who completed before ketamine the following evaluation :

DN4, QCD, QDSA, PGIC questionnaires, HAD scale, Leed's sleep evaluation and SF36.

• Patient who completed at the end of treatment the following evaluation : numerical scale and PGIC
• Sufficient cooperation and understanding to comply to the requirements of study
• Acceptance to give a written concert
• Affiliation at system of French social security
• Inscription or acceptation of inscription at national register of voluntaries participant at research

Exclusion Criteria:

• Against-indication at memantine or dextromethorphan administration : hypersensitivity at active substance or excipients, hypertension, antecedent cerebrovascular accident, severe cardiac insufficiency
• Patient with medical or surgical antecedents
• Patient with progressive disease at balance of inclusion
• Patient treated by an IMAO
• Woman in childbearing age not using effective contraceptive method, pregnant or lactating woman
• Patient who participated in another clinical trial, located in exclusion period or received benefits > 4500 euros during 12 months before the beginning of trial
• Patient with cooperation and understanding insufficiency to comply to the requirements of protocol
• Patient with social protection
• No affiliation at system of French social security