Old topic, recent experience.
I have found the Lidoderm patch effective in reducing sensation on the face where the really painful spots are (upper lip, near the jaw hinges, etc) by cutting up one patch to hit the key areas. I found it much more effective when applied for 3 hours. I did need adhesive tape at the edges, as it is difficult to not move the face.
It did not numb like a dental injection numbs the lips. I can normally handle electrolysis on the less sensitive areas of the face for 1 to 1.5 hours without it any pain reduction aids other than acetaminophen.
The product info shows peak level in the blood at 11 to 12 hours, it is systemic in effect as it does raise the level in the blood. When used for back or neck pain it’s 12 hours on, 12 hours off and up to 3 patches at once.
My partner used these for over 6 months for neck and shoulder pain related to surgery with no observed long term effects.
Cost US $10 per patch.
Advantage, can be self applied, not likely to OD with one patch.
Disadvantage, very slow absorption, does not provide full numbing of the area where applied (it was not designed to do that), stays in your system for hours, and as Michael stated, a small amount of injected lidocaine where it is needed will be more effective and will metabolize out quicker.
It would be handy to have a different product that delivered a smaller amount of the drug at a very high rate.