Predicting serotonin toxicity in serotonin reuptake inhibitor overdose

Abstract

Aims

We aimed to investigate the frequency of serotonin toxicity following overdose of antidepressants that inhibit serotonin reuptake and the factors that influence the probability of serotonin toxicity occurring.

Methods

This was a retrospective cohort study of overdoses that included selective serotonin reuptake inhibitors (SSRIs) (70%) and serotonin norepinephrine reuptake inhibitors (SNRIs) (30%) admitted to a tertiary toxicology unit over 23 years. A multivariate mixed effects logistic regression model using NONMEM (7.2.0) was used to determine factors that influenced the probability of serotonin toxicity occurring.

Results

There were 1978 overdoses in 1520 patients; median age 33 y (range: 13–86 years) and 64% female. Median defined daily dose equivalent (DDD) was 15 (1–420). Co-ingestants were taken in 1678/1978 (85%) overdoses: 11 co-ingested the monoamine oxidase-A inhibitor (MAOI) moclobemide, 99 (5%) co-ingested olanzapine, 58 (3%) co-ingested risperidone and 417 co-ingested a benzodiazepine (21%). Serotonin toxicity occurred in 269 overdoses (13.6%). The probability of serotonin toxicity increased slightly per 10 DDD units dose [OR, 1.01; 95% confidence intervals (CIs): 0.93–1.10], increased for an SNRI vs. an SSRI [OR, 1.07; 95% CI: 0.99–1.15], and markedly increased with co-ingestion of moclobemide [OR, 33.12; 95% CI: 7.5–147]. The probability decreased per 10 y age [OR, 0.84; 95% CI: 0.74–0.95], and with co-ingestion of the serotonin 2 A receptor (5-HT_2A) antagonists olanzapine [OR, 0.40; 95% CI: 0.17–0.94] or risperidone [OR, 0.13; 95% CI: 0.02–0.99]. The probability of serotonin toxicity was 12.5% at 1 DDD (therapeutic), 12.7% at 15 DDDs and 19% at 420 DDDs. In overdoses of the median dose of 15 DDDs, co-ingestion of moclobemide increased the probability to 83%, and co-ingestion of olanzapine or risperidone decreased it to 5.5% and 1.8%, respectively.

Conclusions

Serotonin toxicity is common following SSRI/SNRI overdose. Although dose increases probability, this was only a small effect. Co-ingestion with olanzapine or risperidone reduced the risk 2–6-fold, and moclobemide increased the risk 5-fold.

Keywords:

• Serotonin toxicity
• serotonin syndrome
• antidepressant
• overdose
• SSRI
• SNRI
• poisoning

Acknowledgements

The authors acknowledge the assistance of the medical and nursing staff at the Calvary Mater Newcastle emergency department. We also thank the Clinical Toxicology Research Group research staff members for data entry, maintenance of the toxicology database and for obtaining medical records for study participants.

Author contributions

GI, JC and SD designed the study; GI and JC identified patients; GI and JC did the data extraction; JC carried out the analysis of the data; JC did the literature review; JC drafted the manuscript. All authors read and approved the final manuscript. GI is guarantor of the article.

Disclosure statement

All authors have completed the Unified Competing Interest form at http://www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.

Additional information

Funding

Geoff Isbister is funded by a National Health and Medical Research Council (NHMRC) Senior Research Fellowship ID1154503.