Chemotherapy drugs like doxorubicin, which is used to treat breast cancer, cause serotonin to be released in the gut, leading to nausea and vomiting.

Taking lithium along with meperidine (Demerol) might cause too much serotonin in the brain and serious side effects including heart problems, shivering, and anxiety.

Taking lithium along with pentazocine (Talwin) might cause too much serotonin in the body.

Taking lithium along with tramadol (Ultram) might cause too much serotonin in the brain causing confusion, shivering, stiff muscles and other side effects.

Taking L-tryptophan along with tramadol (Ultram) might cause too much serotonin in the brain and side effects including confusion, shivering, and stiff muscles could result.

Taking St. John's wort along with some medications for migraine headache might cause too much serotonin in the brain and serious side effects including confusion, shivering, stiff muscles, and other side effects.

It is thought that when Gammarus lacustris is infected with a thorny-headed worm, the parasite causes serotonin to be massively expressed.

Taking SAMe along with these medications used for depression might cause too much serotonin in the body, and serious side effects including heart problems, shivering, and anxiety.