What is it that you're scared of?
Getting clear on that (I mean like writing it down, so you can get it out of your head and so that it forces you to be very clear in defining the issues) might be a good first step, because then you can realistically assess the risk and likelihood, and make plans about what you would do if [whatever it is] did happen.
Plus, writing it down means you can do it in short bursts, if that suits you better. And being able to look at it changes it from "argh, scary nameless fear" to "set of things I can deal with one by one".
Have you looked into researching the meds you've been prescribed (lots of info online, from reputable sources), so that you can assess how realistic your fears are?
Once you've done the research, if you were still concerned, could you speak to your doctor or healthcare team to discuss your fears?
Also, anxiety often goes along with ADHD, so if that's relevant for you, you might find that CBT could be helpful in addressing those kinds of thought patterns, as could having someone to talk it through with.
You went to the effort to get a diagnosis, so presumably there are issues that you did want to address? It might also be worth drawing those thoughts/factors in - what positive outcomes might arise if the meds work as intended?
Good luck!