This I learned from my own first hand observations; Vitamin C will reduce the effectiveness of anesthesia.
If you are okay with accepting the general premises of A) anesthesia consituting a form of toxin and B) Vitamin C possessing powerful (fast too!) anti-toxin capabilities among it's various benefits, then you understand the recent dental phenom you experienced.
It took me a good while, over the period of several years of unwtittingly trialing & erroring this principle. I still remember one particulary nasty crown-prep that turned out exceptionally painful. Crown prep in this instance involved doc dentist grinding , shaving and shaping a natural molar down to a nub as step 1 in prep for step 2 - installing an artificial crown on top of that nub. In this instance, the severe pain that set in after step 1 required an emergency step 1A - impromptu root canal procedure. This same pattern would end up repeating itself for me near identically on 2 near-future crown/RC events. After jabbing with novacaine as foreplay to the rooting out of the nerves in the canals, I distinctly noticed the sensation of pain while doc commenced with the rooting out procedure. Doc distinctly noticed my immediate reaction to his rooting out ministrations and after a bit of "hmmmmm?" head scratching, he reapplied more novacaine - 3 MORE TIMES, separately, before reactions to sensations were finally dumbed & numbed enough for him to proceed.
At this time I still was not congizant of the Vitamin C - Novacaine connection BUT I would eventually figure ot that this was one experience that eventually lead me to be aware of this connection. During this same period, I had for years been a reguarly daily morning Vitamin C megadoser, and on the very morning in question had consumed the routine daily megadose within the hour prior to visiting dentist for this emergency procedure. It would still be another year or two, that also involved the process of finding a new / alterna dentist, and his help / knowledge / familiarity with the Vitamin C /Novacaine connection that the light finally came on for me.
Personally, I have found the advice about "lay off Vitamin C for a week" to be way overkill. When alternate dentist was removing all my Amalgams and root canals, one by one, he advised me ahead of time that I simply put off consuming that day's Vitamin C until AFTER his work. What I adapted to was preparing a megadose in a small jar, taking it with me to dentist, letting it sit in car, and then the first thing I did immediately after each visit was drink the C. This particular regimen seemed to work fine, but then again, I'm just one person. Your mileage may vary AND there may be more to this "1 week" advice that seems to be growing up around the C / Novacaine phenom.