My ex girlfriend's new boyfriend have a history of drug abuse should I be concerened for my daughters safety?

My ex girlfriend and I have a 9 year old daughter we have joint custody of her she lives with me during the week and my ex has her during the weekends. My ex is dating a former drug addict he says hes clean he was in rehab for 9 months. My ex told me i have nothing to worry about I threatened him not to do drugs around my daughter or else. My ex say she would never put our daughter in harms way and that I can trust hher. I second that should I be worried about my daughter staying over there?