Despite a massive increase in the number of antidepressants being prescribed to Australians, in the past few decades, neither the rates of self-harm nor the severity of antidepressant poisoning have increased, according to research published in the Medical Journal of Australia.

A 26-year study, from 1987 to 2012, of admissions data from a toxicology centre serving Newcastle, Lake Macquarie and Port Stephens, in New South Wales, found that although there was a more than six-fold increase in antidepressant use, there was only a very modest increase (approximately 1.34-fold) in the proportion of poisonings due to antidepressants.

In addition, the median length of stay (LOS) in hospital decreased from 20.5 hours in the first 6 years of the study to about 16 hours in the second 5 years and remained at that figure for the rest of the study period. The fatality rate dropped from 0.77% to 0.17%, the rate of admission to the intensive care unit (ICU) dropped from 19.2% to 6.9%, and the rate of mechanical ventilation dropped from 13.7% to 4.8%.

Of the 17,266 admissions of patients, most (88.8%) were attempts at self-harm and the remainder were unintentional and recreational self-poisonings.

The median age of admitted patients was 32 years (range, 14-97 years) and there was a predominance of females.

Most (55.4%) of the 17,266 admissions were of single patients; less than a quarter (23.8%) were married patients and very few were either separated or divorced patients (about 8% each).

"The most commonly ingested substances were benzodiazepines (15.9%), alcohol (15.9%), paracetamol (13.5%), antidepressants (13.0%), antipsychotics (9.3%), anticonvulsants (4.4%), opioids (3.6%), non-steroidal anti-inflammatory drugs (3.2%) and antihistamines (2.2%)", the authors wrote.

"Prescription items accounted for 18,950 agents (55.2%), of which 14,445 (76.2%) were known to have been prescribed for the patient."

Lead author Professor Geoffrey Isbister, from the University of Newcastle's School of Medicine and Public Health, and his co-authors wrote that changes in the kinds of antidepressants being prescribed (from more to less toxic) was a significant factor in the rates of self-poisoning.

"Despite a large increase in prescriptions for drugs used to treat psychiatric illness (and a range of other major mental health interventions), there appears to have been no positive result in terms of reducing episodes of self-harm", the authors concluded.

"Interestingly, there was a more than sixfold increase in the use of antidepressants, and while the agents taken in overdose changed substantially, there were only small changes in rates of antidepressant overdoses.

"Interpreting this rather surprising finding is not straightforward.

"It probably indicates that antidepressants are increasingly being prescribed for patients who have minimal risk of self-harm. Reassuringly, there is no evidence in our population to support concerns about pro-suicidal effects of new antidepressant prescriptions.

"The lack of any change in overall self-harm rates also suggests that increased antidepressant use for depression is not an effective public health strategy to reduce the rates of self-harm."

The authors also suggested that there was a need for coordinated collection of data about self- poisoning and suicide.

"First, a national coronial register of drug-related deaths is essential to enable an analysis of relative mortality", they wrote.

"Second, data on poisonings reported to poison centres are essential, particularly for childhood poisonings that rarely require admission and for assessing the effect of primary prevention measures.

"Third ... systematic use of clinical databases to record poisoning admissions for cases of poisoning is needed to measure relative clinical toxicity."

"The public health benefits would greatly outweigh the modest costs of enhancing postmarketing surveillance through more widespread systematic collection of poisoning and overdose data."