Son's dad put him on happy pills without my knowledge and I'm worried if I give them to him Friday he won't give my boys back

I had my older kids for winter break. They both live with their dad but see me on weekends and were here for winter break. My older boy who turns 17 next week has been dealing with depression for over a year. He has had normal teenager things happen in the last year. His best friend stopped being his friend. His girlfriend broke up with him. Nothing serious enough to warrant such bad depression. It had been going on for several months. We had him seeing a therapist for self harm but then he started suggesting medication and I said no. I don't want my kids on happy pills. I told my ex straight up that I didn't. My boys were here and my oldest was fine. We had fun. But my 3 year old went through his bag. I scolded her but she was trying to open a pill bottle. He was prescribed Zoloft. So obviously I was really unhappy. I called his dad and his dad said he's been on it for 3 months and is doing better. I got mad because I didn't want my child on happy pills and he did this behind my back. I flushed them down the toilet and his dad cussed me out about it and called me horrible names. My husband told me I shouldn't have done that and he could possibly use this against me in court if he wanted to take away my overnights but how can he just put my child on happy pills without my consent? My son is also mad at me even though I explained to him he doesn't need happy pills. I'm supposed to give my boys back Friday but I'm scared when I do my ex is going to fight to not give them back. What do I do?

@Melissa and happy pills can destroy his liver. Have you not read all the side effects of antidepressants? I can monitor my child to make sure they aren't committing suicide however I can not monitor his liver. Also, I'm a CNA. I work in the medical field thank you

@Nadia I never said I wanted control over my child. Some people claim crack helps them feel better. Are you going to let you child smoke crack? I had put him in therapy to help with his depression.

Edit: Wow. The misogyny is real with these comments. You all calling me a part time parent and hating on me for their dad having full custody when you know nothing about that but when a dad has his kids twice a month and pays child support nobody bats an eye. In fact a woman should be lucky to have a man who pays child support and gets his kids at least sometimes. Whereas I see my kids every week. Just on weekends and I'm a shitty part time mom. Some of these comments reek of internalized misogyny. I'm a mother. I carried them. I know the dangers of antidepressants. He should not have given those to him when I said no