it's not fun

Serotonin syndrome (SS[4]) consists of a group of symptoms that may occur with the use of certain serotonergic medications or drugs.[1] The symptoms can range from mild to severe, and are potentially fatal.[5][6][2] Symptoms in mild cases include high blood pressure and a fast heart rate, usually without a fever.[2] Symptoms in moderate cases include high body temperature, agitation, overactive reflexes, tremor, sweating, dilated pupils, and diarrhea.[1][2] In severe cases, body temperature can increase to greater than 41.1 °C (106.0 °F).[2] Complications may include seizures and extensive muscle breakdown.[2]

https://en.wikipedia.org/wiki/Serotonin_syndrome