Intentional drug overdose is an important cause of Emergency Department attendances in the United Kingdom and elsewhere 1. Antidepressant agents are commonly implicated in drug overdose, and may result in significant morbidity and mortality 23. Clinical outcomes after intentional antidepressant overdose vary substantially depending on the type of agent involved and dose ingested. For example, monoamine oxidase inhibitor (MAOI) ingestion can result in accumulation of norepinephrine within the central nervous system, and may cause profound haemodynamic instability, seizures, coma and death 45. Tricyclic antidepressants are capable of blocking sodium channel conductance such that overdose is characterised by the occurrence of life-threatening arrhythmia, severe hypotension, generalised seizures, coma, and death 67. In contrast, overdose involving a selective serotonin reuptake inhibitor (SSRI) is characterised by less overall toxicity, fewer life-threatening complications, and a lower mortality than that attributed to MAOI or tricyclic antidepressants 8910.

In recent years, there has been a trend towards increase antidepressant prescribing due widened clinical indications for pharmacological treatment. Prescribing patterns have changed such that MAOIs and tricyclic antidepressants are less widely relied upon, and there has been a substantial increase in the use of SSRIs and novel agents including duloxetine, mirtazapine, and venlafaxine 1112. These changes have been prompted by several factors, recommendations in clinical guidelines, adverse effect profile of specific drugs, marketing activity by the pharmaceutical industry, patient preference, and the perceived risk of self-harm in individual patients. Many existing pharmacovigilance systems have been designed to allow collection of data concerning medications used in accordance with the Marketing Authorisation, namely adverse effects arising from intended therapeutic use including prescribed dosages. In contrast, data concerning adverse outcomes in the setting of intentional drug overdose is not collected in a systematic manner, and these data tend to emerge as individual case reports, published case series, and abstracts at scientific meetings. Comparatively few data are collected systematically in this patient group, and there is a need for a better understanding of the link between exposure and extent of exposure to particular drugs and clinical outcomes. The present study sought to examine the possibility of using Emergency Department data to identify patterns of antidepressant overdose with respect to prevailing prescribing practices, and to evaluate clinical outcomes between specific agents as a measure of toxicity.

There were 1092 Emergency Department attendances due to intentional drug overdose, including 250 episodes that involved an antidepressant (22.9%). The median age was 37 years (population range 16-85 years), and there were 165 women (66%) (Table 1). Hospital presentations involved an SSRI in 183 (73.2%), a TCA in 45 (18.0%), and another antidepressant in 22 (8.8%). Patients that ingested a TCA were more likely to require admission to hospital, had a longer duration of hospital stay, and were more likely to require transfer to a critical care area (Table 1). One death occurred in a 34-year-old woman with a past history of chronic alcohol excess that presented to hospital due to asystolic cardiac arrest after overdose involving unknown quantities of escitalopram, paracetamol, and codeine. Patterns of antidepressant overdose in men and women are presented in Table 2.

There were 1,304,795 prescription items for antidepressants in the York and North Yorkshire region, and these included 749,144 (57.4%) items for a SSRI, 427,127 (32.7%) for a TCA, and 129,243 (9.9%) others. The overall occurrence of antidepressant overdose was equivalent to 19 episodes per 100,000 prescription items (95% CI 17-22 per 100,000 scripts). A higher number of overdoses were observed in the SSRI group compared to the TCA group: 24 (95% CI 21-28) versus 11 (95% CI 8-15) respectively (P<0.0001). Citalopram was the most widely prescribed, and used as a reference comparator (Table 3). Fluoxetine was associated with a higher number of overdose episodes: 32 per 100,000 scripts (24-41) versus 22 (17-27) respectively (P=0.032). Fewer than expected overdose episodes involved amitriptyline (13, 95% CI 9-17, P=0.004) and dosulepin (2, 95% CI 0-10, P=0.001) (Figure 1).

Around five and a half thousand deaths were attributed to drug poisoning in England and Wales alone in 2010-2011, and antidepressants account for a high proportion of these. The age-standardised annual mortality rates attributed to antidepressant overdose are 7.1 (95% CI 6.1-8.1) and 6.7 (95% CI 5.8-7.6) per million in men and women respectively, which are almost double those attributed to paracetamol poisoning of 3.1 (2.4-3.7) and 3.6 (3.0-4.3) deaths per million in men and women respectively 17. Within these deaths, differences exist between agents as a result of variations in their accessibility and inherent toxicity 18. Citalopram was the most widely prescribed antidepressant and most likely to be implicated in overdose, despite mounting data that it is capable of prolonging the QT interval and may predispose to arrhythmia 1920. The fatal toxicity index has been proposed as a measure that takes account of the number of prescription items and, for example 21. This indicates between 11 and 35 deaths per million antidepressant prescription items, including 27 to 47 deaths per million amitriptyline prescription items 2223. An alternative approach has been to consider the case fatality, expressed as a ratio of fatal to non-fatal self-poisonings; the case fatality has been reported for tricyclic antidepressants and SSRIs as 13.8 (95% CI 13.0-14.7) and 0.5 (95% CI 0.4-0.7) respectively (P<0.0001) 24. Only one death was observed in the present study, equivalent to 0.8 deaths per million prescription items (95% CI 0.0-4.3 deaths per million), indicating that only a very small proportion of antidepressant poisoning deaths occur in hospital.

As expected, the present findings indicate that overdose involving tricyclics was associated with a greater need for hospital admission, longer hospital stay, and greater need for critical care compared to overdose involving SSRI or other antidepressants. These observations may be explained as a result of differing intrinsic toxicity because the ingested dose was comparatively smaller in the tricyclic group, and there were no significant differences in co-ingested drugs and ethanol 25. Previous reports indicate that intentional overdose involving MAOI or tricyclic antidepressants may result in severe arrhythmia and prolonged generalised seizures that are resistant to conventional therapy 26. In contrast, overdose involving SSRIs may provoke dose-dependent generalised seizures in around 10%, and these are usually self-limiting or promptly terminated by benzodiazepine administration 27. SSRI poisoning gives rise to prolongation of the QT interval in around 5% of patients, but the occurrence of life-threatening cardiac arrhythmia is thought to be very rare 2829.

The study design did not directly address the factors that might give rise to preferential involvement of particular drugs amongst patients that attend hospital after intentional overdose, and a number of possible explanations exist. Firstly, a high level of awareness of the comparative toxicity of various antidepressant agents and judicious prescribing would result in greater reliance upon SSRI agents, and less reliance upon tricyclic or MAOI agents when issuing prescriptions to patients that are considered to be at increased risk of self-harm. Secondly, the tendency towards increasing antidepressant use coupled with SSRIs being relied upon as first-line treatment means that a greater number of ‘low risk’ patients will receive an SSRI, and this group may be more likely to seek medical attention after intentional overdose. Thirdly, tricyclic antidepressants may be prescribed where SSRI treatment has failed, demarcating a subset of patients with more resistant or severe depressive illness that may be less inclined to seek medical attention after overdose. Fourthly, tricyclic antidepressants may have been initiated in the past, and used to maintain long-term remission of symptoms, and this patient subgroup may be less likely to exhibit self-harm behaviour. Lastly, tricyclics may be more likely to be used for disorders such as neuropathic pain, where the risk of intentional overdose is expected to be lower.

SSRIs were more highly represented amongst overdose patients than might have been predicted from the numbers of prescription items, whereas fewer episodes involved a tricyclic antidepressant. These data suggest a preferential use of SSRI rather than tricyclic antidepressants in patients at risk of self-harm. Further work is required to understand the factors that influence antidepressant prescribing so as to minimise the risk of harm in this at-risk patient population. These findings indicate that Emergency Department data are sufficiently sensitive to detect different clinical outcomes between antidepressant agents in overdose. These data may make a useful addition to the existing sources that are relied upon in conventional pharmacovigilance programmes. Further work is required to explore the feasibility of systematic clinical data collection and how these data might be used to inform prescribers in order to minimise adverse outcomes in patients at high risk of self-harm.