Comparison of the Fatal Toxicity Index of Zopiclone with Benzodiazepines

Abstract

Background. Zopiclone is a hypnosedative structurally unrelated to the benzodiazepines but operating at the same receptor complex. Although zopiclone has been used in clinical practice for many years, relatively little is known of its relative toxicity in comparison with other hypnosedatives. Method. Deaths, where hypnosedatives were implicated, in New Zealand (NZ) in 2001 were identified from a chemical injury database. Prescription and aggregate defined daily dose (DDD) data for NZ in 2001 were obtained from a national prescribing database. Rates of death per prescription and DDD, and relative rates between individual hypnosedatives and benzodiazepines, and their respective 95% CI were calculated Results. Of the 200 poisoning deaths in NZ for 2001, 39 involved hypnosedatives, and zopiclone was involved in 12. Hypnosedatives were the sole agents in only one death and were the primary agents in eight deaths. Zopiclone was the sixth most commonly involved agent in poisoning deaths in NZ in 2001. The relative rate of death per prescription (95% CI) and DDD (95% CI) of zopiclone compared with benzodiazepines were 1.04 (0.49–2.05) and 0.59 (0.28–1.16), respectively. The relative rates of death per DDD (95% CI) for alprazolam and chlormethiazole compared with the other sedatives/anxiolytics were 6.2 (1.6–17.0) and 20.9 (2.5–79.8) respectively. Conclusions. The fatal toxicity for zopiclone was not significantly different from that for benzodiazepines as a group when adjusted for usage, whereas alprazolam and chlormethiazole had greater toxicity. Hypnosedatives are contributory factors rather than primary substances in poisoning deaths.