Can I sue a dentist for using bad fillings?

While dentists (or any medical professional for that matter) can certainly be held liable for medical malpractice, most often it's difficult to bring a suit against them becasue the damages are usually too small.

From a legal sense, it sounds like you may have a valid dental malpractice claim, however, you have to consider the value of your injuries. If you require two root canals, and new fillings, your total medical expense may be around $10,000 or less. You can certainly file a suit demanding recovery of your expenses, but it's unlikely that the suit itself would cost less than that to prosecute.

It's a question of the mathematics of a medical malpractice lawsuit. If the value of the settlement is less than that cost of pursuing it, it will be hard to get a lawyer to take your case. Now, there could be other damages for which you could seek additional compensation, so it's always a good idea to talk to a lawyer and explain all the deatils of your case.