Serotonin syndrome may result from the combined use of dextromethorphan and serotonergic antidepressants such as selective serotonin reuptake inhibitor (SSRIs) or monoamine oxidase inhibitor (MAOIs). Further research is needed to determine whether doses of dextromethorphan beyond those normally used therapeutically are needed to produce this effect. In any case, dextromethorphan should not be taken with MAOIs due to the possibility of this complication. Serotonin syndrome is a potentially life-threatening condition that can occur rapidly, due to a buildup of an excessive amount of serotonin in the body.
Patients who are taking dextromethorphan should exercise caution when drinking grapefruit juice or eating grapefruit, as compounds in grapefruit affect a number of drugs, including dextromethorphan, through the inhibition of the cytochrome P450 system in the liver, and can lead to excessive accumulation of the drug which both increases and prolongs effects. Grapefruit and grapefruit juices (especially white grapefruit juice, but also including other citrus fruits such as bergamot and lime, as well as a number of noncitrus fruits) generally are recommended to be avoided while using dextromethorphan and numerous other medications.