Is it very painful to fill cavities without anesthetics?

Shallow are painless: Many time a shallow cavity or gum line erosion can be filled without anesthetics. Otherwise, unless you have a high tolerance for pain, take the shot.

Depends: It depends on the depth of the decay, and of course, patient tolerance. Shallow decay, if caught early can often be treated without any anesthetic, and without any pain. For deeper or more complex dentistry local anesthetic is recommended, as it is usually best for the comfort of both the patient and the dentist!

Maybe: It really depends on the person, their age, and he depth of the cavity. Some can have most everything done in their mouth w/o anesthetic, and others need it even with minor procedures.