In this study, the prevalence of participants reporting increased substance use was 28.7%. Even though more men than women used ONSET, the female and male responders did not significantly differ in the prevalence of increased substance use. However, the results suggest that gender specific substance use exists. While women reported an increased use of pharmaceuticals, men reported increased use of alcohol and cannabis. Stratified multivariable regression analyses showed a multivariable association between any substance use and the severity of PTSD symptoms in both genders.
In women not only the PTSD symptoms but also the number of different exposures to the incident were significantly associated with increased alcohol use, increased consumption of pharmaceuticals, and increased consumption of cannabis, but not with increased tobacco consumption. In the male strata, the increased substances use was associated only with the severity of PTSD symptoms.
Overall, our study confirms the finding of increased substance use after traumatic experiences [14, 17–22]. However, one limitation of the study is that we did not further explore the reasons for increased substance use. While some individuals might have used the substance as a self-medication, others might have consumed a substance as a coping mechanism. This would be the case if a person, who was emotionally distressed by the incident, sought relief in a behavior that has previously helped to reduce their level of distress. Even though our results do not permit us to answer this question directly, the finding from previous studies [13, 25], that the number of people who consume specific substances does not increase, but that the amount consumed per person does, suggests, that the increased substance use is rather a coping mechanism than self-medication.
A second concern is that while increase of substance use was assessed it was not taken into consideration that persons might respond with a decrease in substance use. Thus no final inference is possible as to an overall increase of substance use after exposure to the Tsunami disaster.
A second limitation of the study was that we could not control for prior traumatogenic life events. Since ONSET was primarily designed as a public health screening instrument and not as a scientific research questionnaire, we tried to keep the instrument as short and user friendly as possible. Hence we did not explore pre-existing traumatic experiences or substance consumption habits at all. Furthermore, the data are cross-sectional and it is therefore impossible to determine whether the self reported mental health problems are the results of the Tsunami or represent the exacerbation of pre-existing problems.
A third concern is the composition of the study sample. Although the present study sample is of considerable size, the study population was a convenience sample and it was not possible to assess to what extent and with regard to which characteristics our sample was biased (when compared to the whole population of people affected by the tsunami). In order to use ONSET, the availability of the instrument had to be known, potential users needed Internet access, an email account, and basic computer skills. However, the results of our study are not meant to be interpreted as estimates of increased substance use in the general population, but rather serve as an estimate of online users in Switzerland.
A fourth limitation is, that we were not able to perform structured clinical interviews, but had to rely on self-reports. Also, the data were strictly anonymously assessed. Whether this increases or decreases the validity of the data in this context, cannot be answered. However, when analyzing the descriptive results, there were virtually no striking outliers such as unusually old age declarations that could have been interpreted as voluntary or involuntary false and misleading statements. Our results correspond to the findings of other studies in the field and we did not identify any response behavior that could be interpreted as a limitation of the internal validity of ONSET. We consider that there is a certain degree of empirical evidence that supports the overall validity of the study design and hence supports the use of an online assessment instrument for public mental health intervention strategies – especially when time matters.