My mother drugged my drink so I would fail a drug test. How can I prove it?

I had my suspicions of her doing it before, now I'm sure. We were in a Starbucks in Barnes and noble, she offered to get me a drink, first asking if I was testing again. I got a video of her doing it but the cops said it wouldn't prove anything- I called them right after she did it and tried to get them to test the drink- they said even if they did find anything there wouldn't be any proof that she put it in there.

There were no witnesses and no video. I saw it with my own eyes. She's dangerous and no one will believe me. Is there anything I can do at this point? I've already scheduled a lie detector test, but that only helps my family believe me, it won't help my case against her. Now that the cops talked to her I know she won't try it again, I don't live with her so I don't need to worry about her poisoning my food or anything. Any ideas would be helpful.