What are my rights with shared legal custody?

I am a divorced father of three living in Pennsylvania. I have shared legal custody and partial Physical custody of my three daughters. My question is, what does "shared legal custody" entitle me too? My ex wife has my youngest daughter involved with Children Services and taking a narcotic behavior modification drug. I disagree on my daughter seeing this doctor OR taking the medication, as I feel the only problem my daughter has is a mother whom prefers to spend the majority of her day playing games online. When the child is in my custody, She acts like what I would consider a healthy, active seven year old girl.