Derek's metal jaw makes it harder for him to eat due to the sharp metal and lack of teeth in the bottom. His gums hurt because of the fake teeth and he gets toothaches because of it. He has to avoid certain foods because of this. If he doesn't take painkillers he will end up suffering and laying on the floor complaining how his jaw hurt.
Painkillers are a huge relief tool for him. He always takes his painkillers as much as possible. He hopes one day he will be able to have his jaw replaced with a realistic one than the one he has now so he won't hide his face again. He has to rely on ice packs to soothe the pain from his prosthetic jaw.