Our research involving substance misuse investigates the biological and psychological characteristics of heroin users using heart rate variability (HRV) and the tridimensional personality questionnaire (TPQ). Our results show those with heroin misuse in Taiwan have borderline personality tendencies – low interpersonal dependence and high impulsivity. Those with heroin misuse also have reduced parasympathetic tone, a cardiovascular protective factor. We have also found that those with higher borderline personality traits have higher sympathetic tones and reduced parasympathetic activity. HRV can be used to measure adherence to methadone treatment, and to provide a marker as a risk factor for future cardiovascular illness in heroin users.