the National Committee for Quality Assurance recommends avoiding the use of cyclobenzaprine in the elderly because of the potential for more severe side effects. Dysphagia, a life-threatening side-effect, may rarely occur. Treatment protocols and support should follow the same as for any structurally related tricyclic, such as tricyclic antidepressants. Overdose The most common effects of overdose are drowsiness and tachycardia. Rare but potentially critical complications are cardiac arrest, abnormal heart rhythms, severe low blood pressure, seizures, and neuroleptic malignant syndrome. Life-threatening overdose is rare, however, as the median lethal dose is about 338 milligrams/kilogram in mice and 425 mg/kg in rats. The potential harm is increased when central nervous system depressants and antidepressants are also used; deliberate overdose often includes among other drugs. Interactions Cyclobenzaprine has major contraindications with monoamine oxidase inhibitors (MAOIs). At least one study also found increased risk of serotonin syndrome when cyclobenzaprine was taken with the serotonergic drugs duloxetine or phenelzine. These substances may interact with cyclobenzaprine: Central nervous system depressants (e.g. alcohol, opioids, benzodiazepines, nonbenzodiazepines, phenothiazines, carbamates, barbiturates, major tranquilizers) Monoamine oxidase inhibitors taken within two weeks of cyclobenzaprine may result in serious, life-threatening side effects. Cyclobenzaprine may affect the medications used in surgical sedation and some surgeons request that patients temporarily discontinue its use prior to surgery. Pharmacology Cyclobenzaprine is a centrally acting muscle relaxant. Cyclobenzaprine is a 5-HT2 receptor antagonist; it relieves muscle spasm through action on the central nervous system at the brain stem, rather than targeting the peripheral nervous system or muscles themselves. Pharmacodynamics Pharmacokinetics Cyclobenzaprine has an oral bioavailability of about 55% and approximately 93% is bound to proteins in plasma. The half-life of the drug is 18 hours and it has a plasma clearance of 0.7 litres per minute. Comparison to other medications Cyclobenzaprine has been found to be not inferior to tizanidine, orphenadrine, and carisoprodol in the treatment of acute lower back pain, although none have been proven to be effective for long-term use (beyond two weeks of treatment). No differences in pain or spasm scores were noted among these agents, nor when compared to benzodiazepines. However, nonbenzodiazepine (including cyclobenzaprine) treatment was found to have a lower risk of medication abuse and continuation of use against medical advice. Side effects such as sedation and ataxia are also less pronounced with nonbenzodiazepine antispasmodics. In a study on the treatment of musculoskeletal pain treatment with cyclobenzaprine alone or in combination