Non custodial father is medicating daughter during visits every other weekend

I have a 6 year old daughter. Her father and I share legal custody and I have physical custody. She visits him every other weekend. This has been the situation since she has been 2 years old. I recently found out from my daughter this past summer that her father has been giving her 'cherry medicine' every night during her two night stay. I questioned him and he denied doing this. I can't imagine why she would make this up. He has a track record of lying. She also returns home wearing the same clothes each weekend - clothes from his house, but the same skirt, etc. She told me that she has clothes, buther dad makes her wear the same clothes the whole weekend. I am frustated and lost. What can I do to make sure she is okay? I asked her why she gets medicine and she tells me various answers: dad says he has allergies and doesn't want me to get allergies, my nose was stuffy, etc. What can I do if he lies when I question him?