Epigenetics refers to the genomic response to environmental influences. These may be considered in correlation or interaction capacities using qualitative or quantitative methodological approaches. You would most likely want to look at changes in methylation patterns between say non-responders and responders or ADRs vs no ADRs (since you asked about drugs) together with a particular environmental influence (e.g. childhood adversity) to see if methylation influences treatment outcome in terms of (in this case) severity of childhood trauma.