First of all, I will assume that your employers did a background check on you, which I believe is the law if you are going to be responsible for other people, most especially the mentally handicapped. If any of your past drug use involved the police or left you with a record, then they (your employer) knows and has accepted and trusted you with your current position. If they do NOT know, how do the people you are taking care of know? My only guess is that YOU have told them. If you do feel that your employment may be at risk due to this knowledge, you owe it to yourself AND your employer to sit down and discuss this issue. I most certainly am NOT saying you lied to your employer, but if there is something in your past which could jeopardize their company or your job, it is your duty to get everything out in the open. I don't know if you have to have a license to do what you do, but you don't want to risk losing that. That you HAD a problem and overcame it only makes you a more valuable and compassionate employee.
My advice, talk to your employer and get this all straightened out.
Peace
Greenlydia