Serotonin Syndrome Presenting with Concomitant Tramadol and Diphenhydramine Use: A Case Report of an Unlikely Side-Effect

: Serotonin syndrome is a condition that occurs following the administration of serotonergic drugs. Interestingly, on rare occasions, it can occur with various drug combinations that can secondarily affect the serum levels of 5-hydroxytryptamin. Tramadol is an analgesic that has mu opioid receptor agonist activity and has also been shown to inhibit the reuptake of serotonin and noradrenaline. Diphenhydramine is a first-generation histamine antagonist prescribed to treat or prevent allergic reactions and can also be used as a sleeping aid. Here, we demonstrate a case of serotonin syndrome following the administration of diphenhydramine for seasonal allergies in a patient on tramadol for neck pain.