Absolute worst case you could always keep track of it as a raw text/markdown/excel/Libre calc/whatever your preference is. You’re not going to get any predictions or useful data out.
But it would at least provide a record for your doctor if need be. And as long as you encrypt the device you store it on, or the directory its stored in, it’s relatively safe to do so.
A potentially good option if you’re on Android is putting it in your secure folder. It’s basically just a sub directory for all your files, notes, and pictures, but it an encrypted form.
So if you’re technical enough to understand jow to enter a password (most people), then you have an easy to use option. Just don’t forget your password, and don’t set it to something easy to crack. It’s the same rules for any other password.
I’m sure there are similar options for windows/mac/ios
But if you’re a more technical user, by all means it is in your interest to encrypt the whole thing.
https://lemmy.world/post/21961202