Mental health: Managing side effects (part 2)

Last updated: Monday, April 29, 2024

Serotonin syndrome

Medicines that boost the activity of serotonin can cause this syndrome, often when two or more are taken together but sometimes when large doses of a single serotonergic medicine are taken. Some common medicines known to cause serotonin syndrome include most antidepressants, St John’s Wort, tramadol, triptans, pethidine, and lithium. The resulting excess of serotonin causes a range of symptoms affecting muscles, mental state, and the autonomic nervous system as illustrated below:

Serotonin syndrome shares many symptoms in common with neuroleptic malignant syndrome (NMS) which, as its name suggests, is caused by antipsychotic (neuroleptic) medicines. Whilst the serotonin syndrome is caused by pro-serotonin medicines, the neuroleptic malignant syndrome is precipitated by dopamine antagonists. This can be an important way of differentiating between the two syndromes as they both can cause symptoms such as sweating, tachycardia, and changes in mental state. Another important difference is that serotonin syndrome tends to cause muscle spasms (clonus), whilst NMS typically presents with extreme muscle rigidity. 

In practice NMS is very rare and diagnosis should only be made when other circumstances (e.g. infection alongside extrapyramidal symptoms) have been ruled out. This is because future antipsychotic treatment becomes very difficult once a diagnosis of NMS has been made.

Other important side effects    

Another example of a group of related adverse effects is the Syndrome of Inappropriate AntiDiuretic Hormone secretion (SIADH). Antidepressants, antipsychotics and carbamazepine are amongst the many medicines that can stimulate the over-secretion of antidiuretic hormone which causes significant fluid retention, resulting in hyponatraemia. This triggers symptoms such as weakness, weight gain, cramps, vomiting and anorexia. It can lead to confusion, coma and death so it's important to pick this problem up early. 

Dopamine inhibits prolactin release and so dopamine antagonists such as antipsychotics have the potential to increase prolactin plasma levels. Hyperprolactinaemia may lead to unwanted milk production and disturbed menstrual periods, and is associated with reduced bone mineral density and increased risk of fractures in the longer-term. 

Weight gain is common with many psychiatric medicines and this together with a severe and enduring mental health condition such as schizophrenia or depression can predispose to diabetes. Additionally olanzapine is known to contribute to insulin resistance. Collectively this patient group is prone to the adverse outcomes associated with metabolic syndrome and so attention to regular physical healthcare is to be encouraged.

At this stage of your career, you're not expected to remember all the details of the various syndromes that we've presented in these pages, but it is important to be aware that they may occur and to be on the alert for patterns of side effects when caring for patients who take medicines for mental health.