Background Pesticide ingestion is among the most commonly used suicide methods globally. Underestimation of deaths by pesticide self-poisoning is thought to be common but under-researched. We investigated potential misclassification and underestimation of suicide by pesticide poisoning and their impact on trends in pesticide suicide in South Korea.

Methods We compared sociodemographic profiles between deaths certified as suicide, undetermined deaths and injuries by poisoning using pesticides, unspecified chemicals and medicines/alcohol in South Korea (1991–2012), and calculated similarity scores. Joinpoint regression analysis was used to examine time trends.

Results The profiles of deaths classified as pesticide suicides showed generally similar patterns to those for undetermined deaths/injuries by poisoning using pesticides and suicides/undetermined deaths/injuries by poisoning using unspecified chemicals. Over the study period, the number of pesticide suicides (on average 2033 per year; range 623–3530) would increase by 15%, 23% and 31%, when assuming that 20%, 30% and 40% of deaths in alternative cause-of-death categories were misclassified pesticide suicides respectively. Age-standardised rates of pesticide suicide increased more than four times in 1991–2003 but fell by around 50% in 2003–2012. This trend did not change when the assumed proportion of misclassification was 20%, whereas the rapid increase in 2000–2003 would be lessened if misclassification occurred by 30% or more.

Conclusions Pesticide suicides may be underestimated in South Korea as some are possibly misclassified as deaths due to other causes; however, such underestimation would not substantially affect estimated secular trends.