Caffeine is bad for an overactive bladder because caffeine is a diuretic which means that the caffeine can make you pee more.
If your bladder is already overactive meaning you have to already pee too much and too often then drinking coffee or other caffeine drinks can cause you to have to pee more and more.
So while it's okay to drink some coffee when you have an overactive bladder you should avoid too much caffeine when you have an overactive bladder.
I don't have an overactive bladder myself but I do drink coffee that contains caffeine and when I drink coffee with caffeine I have to pee more often.
I usually have to pee within 5 minutes of drinking a small cup of coffee.
So I'm having to run to the toilet or risk peeing my pants within 10 minutes of drinking the coffee.
Coffee is also healthy for your heart in moderation and may even prevent dementia but too much coffee can be bad for you.