What medicines can cause you to fail a drug test?

My dad went to see his parol officer to ask if he could go to Georgia for a family reunion and he wouldn’t let him. Everyone told me it was because the parol officer was new. I didn’t believe that, my dads been out of prison for 10 years so idk why he wouldn’t let him go now. Then I heard that he failed a drug test.

I think he’s on blood pressure medicine but I’m not sure what else he’s on. Can a lot of things make you fail or is he on drugs again?