Why does it take eight or more hours for lidocaine or carbocaine to wear off after my dental work?

Your body. It is totally dependent on how your body metabolizes substance such as local anesthetic. Some people are fine after a couple of hours, some take all day for the anesthetic to wear off.
Mtabolism. Some people's metabolism breaks down the Lidocaine more slowly then others. Also, it depends on how much was used to numb the area. The more used usually the longer it takes to breakdown.

Related Questions

Is it weird for it to take hours for lidocaine or carbocaine to wear off after my dental work?

Mtabolism. Some people's metabolism breaks down the Lidocaine more slowly then others. Also, it depends on how much was used to numb the area. The more used usually the longer it takes to breakdown. Read more...
It depends. On how the anesthetic was given, how much and if over the course of a long appointment. There are other local anesthetics that are designed to last longer. Read more...

I have a true, tested allergy to xylocaine (lidocaine). I've used carbocaine ok for years. Could I develop an allergy to it as well? I need some major dental work and I always worry that I will have a reaction.

Med allergy. If you have not yet, it is unlikely. HOWEVER, we can go for years sometimes without an allergic reaction to something and then for some reason we become allergic. Be sure your dental provider is aware of your allergy and is ready to treat for any allergic reaction. . Read more...
Local Anes Allergy. It is always possible to develop an allergy but the likelihood is extremely small. I have never personally heard or seen a patient allergic to Carbocaine. You can always discuss this with an Allergist and be tested first by small subcutaneous injection prior to procedure. Read more...