Although not recommended I have used a screwdriver before as a chisel when I couldn't find my regular chisel and it worked fine.
However be aware that you could break the handle of the screwdriver especially if the screwdriver handle is made from plastic.
Just be gentle with it and it should work just fine and be safe.
I have a screwdriver too that has a metal piece on the top of the handle that's designed to be used as a chisel and it's designed so that you hit that piece of metal with a hammer and it's safe.
If you need to chisel something enough then I would recommend getting a good chisel set or even just a chisel as it would be much safer and you won't ruin your screwdrivers by hitting them with a hammer.