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Medicines Q&As
Q&A 219.3
What is serotonin syndrome and which medicines cause it?
Prepared by UK Medicines Information (UKMi) pharmacists for NHS healthcare professionals
Before using this Q&A, read the disclaimer at www.ukmi.nhs.uk/activities/medicinesQAs/default.asp
Date prepared: 3rd June 2014
Background
Serotonin syndrome has been highlighted by the widespread use of serotonergic agents, such as
those used to treat depression e.g. selective serotonin reuptake inhibitors (SSRIs) (1). The first
reported fatality from serotonin syndrome was over 50 years ago (2), prior to the availability of SSRIs.
The incidence of serotonin syndrome is not known but has risen as a result of the increased use of
serotonergic drugs and the increasing awareness of this syndrome (3).
Serotonin syndrome can be difficult to differentiate from other medical conditions including,
Neuroleptic Malignant Syndrome, anticholinergic toxicity and malignant hyperthermia (3). Healthcare
professionals need an awareness of the range of symptoms of serotonin syndrome and drugs with
serotonergic activity, which are discussed in this Medicines Q&A. This knowledge will aid early
diagnosis and treatment and reduce patient distress as well as potential morbidity and mortality (3).
For in-depth information on specific interactions and switching between medicines see:
 UKMi Q&A 48 Triptans and SSRI or SNRI antidepressants – is there an interaction?
 UKMi Q&A 71 How should depression be treated in a patient with Parkinson’s disease?
 UKMi Q&A 94 What is the risk of developing Serotonin Syndrome following concomitant use of
tramadol with selective serotonin reuptake inhibitors (SSRIs)?
 UKMi Q&A 150 Switching between tricyclic, SSRI and related antidepressants
 UKMi Q&A 151 Switching between monoamine oxidase inhibitors and SSRI, tricyclic or related
antidepressants
 UKMi Q&A 192 What is the risk of interaction between opioid analgesics and monoamine oxidase
inhibitors (MAOIs)?
Answer
The term ‘serotonin syndrome’ is used to describe significant ‘serotonin toxicity’ and the two terms
tend to be used interchangeably (1,2). Current thinking favours the spectrum concept of ‘serotonin
toxicity’ as a continuum of serotonergic effects. This progresses from side effects through to lifethreatening toxicity, with increasing intrasynaptic serotonin levels. Therefore it is a form of poisoning
rather than an idiosyncratic reaction (4). It is due to a toxic hyperserotonergic state from
hyperstimulation of the brain stem and spinal cord 5HT1A and 5HT2 receptors (5), possibly with more
involvement of the latter (1).
The different mechanisms of causing serotonin syndrome are (3):
 increase in serotonin synthesis
 inhibition of serotonin metabolism
 increase in serotonin release
 inhibition of serotonin uptake
 activation of serotonergic receptors
Serotonin syndrome can occur from:
1. Overdose of a serotonergic agent e.g. in a case series approximately 15% of patients taking
an acute overdose of SSRIs developed moderate serotonin toxicity (1).
1
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Medicines Q&As
2. Drug interactions i.e. when more than one drug affecting the serotonin system or inhibiting the
metabolism of serotonergic drugs is taken (3).
 For example, serotonin toxicity is a potential danger of administering two serotonergic
antidepressants together (6). In cases of serotonin syndrome due to drug interactions, the
most severe cases are seen when monoamine oxidase inhibitors (MAOIs), are taken in
combination with SSRIs, tricyclic antidepressants or venlafaxine. This is because the
mechanism of action of MAOIs is to inhibit the breakdown of serotonin (3,7).
 Many drugs, in addition to SSRIs, can inhibit the cytochrome P450 2D6 and/or 3A4
isoenzymes, resulting in the accumulation of serotonergic drugs that are being used
simultaneously (3).
3. Taking one serotonergic agent alone in susceptible individuals (8).
A complete and accurate medication history is essential when diagnosing serotonin syndrome (3,8),
which usually occurs within a few hours of drug or dose changes, resolving within about 24 hours of
stopping the serotonergic drug(s) (5).
Symptoms
Serotonin syndrome or toxicity results in certain symptoms consisting of a triad of features: alteration
of mental status, neuromuscular abnormalities and autonomic hyperactivity (2,3). These three
features are not necessarily present together in every patient (7). Clinical features can range from
mild to life-threatening (3). Around 40% of patients have mental status changes, about 50% of
patients have evidence of neuromuscular hyperactivity. Autonomic instability occurs in around 40% of
patients (7). Severe cases may result in complications, such as seizures, rhabdomyolysis,
myoglobinuria, metabolic acidosis, renal failure, acute respiratory distress syndrome, respiratory
failure, diffuse intravascular clotting, coma, and death (3).
Table 1. Symptoms of Serotonin Syndrome (3)
Alteration of mental status
agitation
anxiety
disorientation
restlessness
excitement
Neuromuscular abnormalities
tremors
clonus
hyperreflexia
muscle rigidity
bilateral babinski signs
Autonomic hyperactivity
hypertension
tachycardia
tachypnea
hyperthermia
mydriasis
diaphoresis
dry mucous membranes
flushed skin
shivering
vomiting
diarrhoea
hyperactive bowel sounds
arrhythmias
Several systems of diagnostic criteria exist, for example the Sternbach criteria and the Hunter
Serotonin Toxicity Criteria (HSTC). The HSTC are the most accurate and specific criteria and are less
likely to miss early, mild or subacute forms of serotonin syndrome (8).
Hunter serotonin toxicity criteria – in the presence of a serotonergic agent plus one of the following
(3,8):
 Spontaneous clonus
 Inducible or ocular clonus AND agitation or diaphoresis
 Tremor AND hyperreflexia
 Hypertonia AND hyperpyrexia (temperature exceeding 38ºC) AND ocular or inducible clonus
2
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Causative medicines
Serotonin syndrome can be caused by certain drugs alone (usually in overdose) or combinations of
serotonergic drugs (8). Clinicians should be aware of drugs that have serotonergic effects and the
combination of any two should therefore be used sparingly or with great caution (3). Some of these
medicines are in therapeutic groups that would not normally be associated with use in depression, or
psychiatry in general, so their serotonergic effects are not immediately apparent (1). These include
drugs such as linezolid or selegiline, both of which have MAOI activity (3). See Table 2 below for
further examples.
Life-threatening cases of serotonin syndrome may occur, for example with the use of any MAOI (e.g.
phenelzine, tranylcypromine) in combination with SSRIs (1). Washout periods should be observed
when switching between these antidepressants. See UKMi Q&A 151 Switching between monoamine
oxidase inhibitors and SSRI, tricyclic or related antidepressants, for further details on switching from
an SSRI to an MAOI and vice versa.
Table 2. Examples of medicines with potential to cause serotonin syndrome (1,3,8,9)
This list only includes examples of medicines with serotonergic effects. Examples of medicines which
have the potential to cause serotonin syndrome solely via cytochrome P450 interactions have been
excluded.
Therapeutic group
SSRI antidepressants
SNRI antidepressants
MAOI antidepressants
Tricyclic antidepressants
Miscellaneous
Opioids
Parkinson’s disease
treatment
Antibacterials
Anti-cancer drugs
Anticonvulsants
Antiemetics
Antihistamines
Antimigraine drugs
Anti-smoking aids
Anxiolytics
Diagnostic dye
Herbal products
Examples of medicines
Citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine, sertraline
Venlafaxine, duloxetine
Tranylcypromine, phenelzine, moclobemide (reversible MAO-A inhibitor),
isocarboxazid
Clomipramine, imipramine, amitriptyline, doxepin, nortriptyline,
trimipramine
Lithium, trazodone, L-tryptophan, mirtazapine
Pethidine, tramadol, methadone, fentanyl, dextromethorphan,
pentazocine, oxycodone, tapentadol
Selegiline (MAO-B inhibitor), rasagiline (MAO-B inhibitor), levodopa
Linezolid (reversible MAOI activity)
Procarbazine
Carbamazepine, valproate
Metoclopramide, ondansetron, granisetron
Chlorphenamine
Triptans (See UKMi Q&A 48) e.g. frovatriptan, almotriptan, eletriptan,
naratriptan, rizatriptan, sumatriptan, zolmitriptan. Dihydroergotamine.
Bupropion
Buspirone
Methylthioninium chloride (methylene blue) (10) - has MAOI activity (3)
St John’s wort
Summary
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The spectrum of ‘serotonin toxicity’ relates to a form of poisoning, not an idiosyncratic reaction. It
progresses from mild to life-threatening toxicity, with increasing intrasynaptic serotonin levels (4).
Serotonin syndrome is due to a toxic hyperserotonergic state from hyperstimulation of the brain
stem and spinal cord 5HT1A and 5HT2 receptors (5), possibly with more involvement of the latter
(1).
Serotonin syndrome can occur from an overdose, drug interaction or even single drug therapy in
susceptible individuals with a serotonergic agent (1,3,8).
Symptoms consist of alteration of mental status, neuromuscular abnormalities and autonomic
hyperactivity (2,3). Patients do not necessarily have all three features (7).
Medicines potentially causing serotonin syndrome all have some form of serotonergic activity (1).
3
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Medicines Q&As
Limitations
Information on the treatment of serotonin syndrome is beyond the scope of this Medicines Q&A.
This Medicines Q&A includes key examples only of medicines available in the UK with the potential to
cause serotonin syndrome – the list in Table 2 is not exhaustive. This list also only includes examples
of medicines with serotonergic effects. Examples of medicines which have the potential to cause
serotonin syndrome solely via cytochrome P450 interactions have been excluded. It does not
encompass amphetamine and its derivative drugs of misuse, which are also known to cause serotonin
syndrome.
This Medicines Q&A is based on key review papers i.e. not single case reports.
References
1. Buckley NA, Dawson AH, Isbister GK. Serotonin syndrome. BMJ 2014: 348:g1626.
2. Gillman K, Whyte IM. Serotonin syndrome. Chapter 3 in Haddad P, Dursun S, Deakin
B,editors. Adverse Syndromes and Psychiatric Drugs: a Clinical Guide. Oxford: Oxford
University Press; 2005 p 37-49.
3. Volpi-Abadie J, Kaye AM, Kaye AD. Serotonin syndrome. The Ochsner Journal 2013: 13: 533540.
4. Gillman PK. A Review of Serotonin Toxicity Data: Implications for the Mechanisms of
Antidepressant Drug Action. Biol Psychiatry 2006; 59: 1046-1051.
5. Bazire S. Psychotropic Drug Directory: The Professionals’ Pocket Handbook and Aide
Memoire. Aberdeen: Healthcomm UK; 2014 p541
6. Taylor D, Paton C, Kapur S. The Maudsley Prescribing Guidelines 11th Edition. London:
Informa Healthcare; 2012 p273.
7. Thanacoody R. Serotonin syndrome. Medicine 2007; 35(10):556-557.
8. Boyer EW. Serotonin syndrome. In: UpToDate. Wolters Kluwer Health. Last updated 2nd July
2014. Accessed via http://www.uptodate.com on 26th August 2014.
9. Boyer EW, Shannon M. The Serotonin Syndrome. N Engl J Med 2005; 352(11):1112-20.
10. Medicines and Healthcare products Regulatory Agency. Methylthioninium chloride (methylene
blue): Update on CNS toxicity with serotonergic drugs. Drug Safety Update April 2009; 2(9):3.
Quality Assurance
Prepared by
Marc Miell, Lead Medicines Information Pharmacist (based on earlier work by Sandra Hicks and Kate
Pickett), Wessex Drug & Medicines Information Centre, University Hospital Southampton NHS
Foundation Trust.
Date Prepared
3rd June 2014
Checked by
Kate Pickett, Lead Pharmacist – Formulary and Medicines Q&As (based on the Q&A originally
checked by Nicola Watts), Wessex Drug & Medicines Information Centre, University Hospital
Southampton NHS Foundation Trust.
Date of check
23rd September 2014
Search strategy
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Embase (via NICE Evidence Search): exp SEROTONIN SYNDROME/ [Limit to: Publication Year
2007-current and (Publication Types Review) and Human and English Language]
Medline (via NICE Evidence Search): exp SEROTONIN SYNDROME/ci [ci=Chemically Induced]
[Limit to: Publication Year 2007-Current and Review Articles].
NICE Evidence. Accessed via http://www.evidence.nhs.uk/
In-house textbooks
4
Available through NICE Evidence Search at www.evidence.nhs.uk