This study specifically examined the impact of lifetime history of substance use on all cause mortality in people with PD. Comparison between drug users and non-drug users were conducted by using t-tests for continuous variables and chi-square tests for categorical variables. The Kaplan-Meier method was used to assess the distribution of time to all cause mortality in alcohol and cannabis users and non-users; log rank tests were used for unadjusted comparisons of mortality. These survival analyses were stratified by age in order to take account of the differences in the mortality among different age groups (<35 years, 35–55 years). We chose 35 years to separate the two groups because people less than 35 years are unlikely to die from cardiac related diseases. Furthermore, in a paper published on cigarette smoking and mortality using the same sample (Kelly et al., 2011), we separated age groups as <35 years and 35–54 years. Cox regression models were used to examine the association of mortality with the most common substances used (cannabis and alcohol) after adjusting for age (18–35 years and 35–55 years), gender,
