Some prepartions of novocain, used by Massachusetts dentists do contain at least one additional additive . As far as a root canal, I've never experienced any side effects from either the procedure or the injection. Usually, depending on the dentist doing the root canal procedure, the only side effect may be an elevation of the blood pressure.
However, I've noticed that after my GENERAL DENTIST gives me an injection of novcaine ( containing an additive ), there is no increase in blood pressure, and the only effect seems to be a calming ( sedation) effect.
Your response and the duration of this response depends on the amount of novaine+, you've received. With older dentists ( GENERAL DENTISTS), the amount given, may be an overkill, because they're not too experienced in pharmacology or pharmaceutics.
With increased dosage, the sedation effect could last at least 24 hours or longer. But remember, this sedation effect is not due to novocaine itself, but rather due to the additive. If you don't know what's in the novocaine injection, causing the sedation, I'd advise you to avoid driving to and from the dentist. I'd also try to stick with dentists in the 40-50 year old range, who may be more up-to-date than those over 70 years of age.
I'd also be careful if your older dentist is having problems with his/her eyesight, as would be expected at the age of 70 years+.