I have custody of son. father abuses drugs. How can i prevent father from seeing him?

What kind of pill are we talking about? If the visitation is court ordered, the judge thought it safe for the child to spend time with his dad. If dad just gave him an asprin or something like that I would just have a conversation with him and say that any and all medicines will be provided by you and given to him to administer if necessary. If the drug given was illegal or he attempted to harm your child than you need to file for a change in visitation and request an emergency custody hearing right away.

If there is no court order, and you are simply doing visitation informally, you can simply discontinue visitation. However, be prepared to explain yourself if and when he files for custody in court. Note: If there is a court order in affect, you cannot modify visitation without the court's approval.