chlorzoxazone has for many years been marketed for the treatment of pain in skeletal muscle by inhibition of mono- and polysynaptic reflexes in the CNS. The muscle relaxing effect is mediated by inhibitory effects on spinal polysynaptic reflexes. In placebo-designed clinical studies of chlorzoxazone's beneficial effect on heterogeneous groups of patients with spasticity, motor neuron syndromes, as well as muscle pain and spasm of peripheral musculoskeletal diseases have not been able to demonstrate no significant analgesic effect; chlorzoxazone have also failed to show pain-relieving effect in the treatment of back pain It is remarkable that in spite of the widespread use of chlorzoxazone not can be found only one study of chlorzoxazone used as adjuvant pain relieving treatment after hip or knee surgery (or other orthopaedic treatment). The current clinical practice is therefore made on purely empirical basis. The effect must be considered as uncertain, but may in some cases be indicated, most often as an adjunct to other therapy, for example analgesics, anti-inflammatory agents, physiotherapy or even training. In such cases, regarded chlorzoxazone be an alternative to benzodiazepines. There is therefore a need for a prospective, randomized, double-blind, placebo-controlled study evaluating the potential analgesic effect of chlorzoxazone. In this intervention study is the selected dosage of chlorzoxazone set at based on the recommendation of the Danish Medicines Information A / S 7 (http://pro.medicin.dk/Medicine / Preparations / 638) and Takeda Pharma A / S. The latter produces chlorzoxazone. The recommendation is based on our current knowledge of chlorzoxazones pharmacodynamic and pharmacokinetic properties. chlorzoxazone given in this study as tablet chlorzoxazone, 250 mg, 3 times daily for the first seven postoperative days. Adverse reactions to chlorzoxazone are well known. They are relatively few, mostly mild and transient. The most frequent adverse events related to chlorzoxazone, fatigue and dizziness (about 1-10% of patients) Adverse reactions are to some extent overlapping with the side effects that are related to the perioperative opioid treatment. It is therefore possible that the frequency of adverse events overall is reduced if chlorzoxazone found to be analgesic (and opioid-sparing). Patients will be hospitalized for a minimum of two nights after surgery and thus be close observation in the period in which the risk of side effects is greatest. At the same time, all patients undergo standardized adverse event registration. If, contrary to expectations unexpected or unacceptable side effects medication will promptly be interrupted. The study involves a group of patients (THA and TKA), which is an important clinical pain problem. All of these patients receive a well-implemented and evidence-based treatment. Thus, anesthesia, analgesia and surgical procedure standardized for all patients receiving concomitant consequences usual principles of early mobilization 8; 9th The planned randomized, double-blind, placebo-controlled design in which all patients receive the same and standardized interventions, enabling the best possible evaluation of modality under study intervention with chlorzoxazone vs. placebo). The level of pain after surgery as the primary endpoint, recorded by means of a well-tested and validated tool (visual analog scale (VAS)) 48 hours after operation. The current study could contribute to a clarification of the postoperative analgesic effect of chlorzoxazone immediately after THA and TKA. As secondary endpoints, the patients functional level be determined Oxford Hip / Knee Score 7 days after surgery. Furthermore, a number of tertiary parameters related to pain, function and side effect profile be determined as shown in Table 1. Evidence in this area will be of immediate benefit to patients operated with THA and TKA - and maybe for other surgical patients.