One of the most common questions I see both in private practice and on this site relates to how long you will be numb after a dental procedure. In dentistry, there are a lot of factors that affect the duration, ranging from the type of the anesthetic to the location given, so there is no one correct answer.
In general, and this is a very broad generalization with many exceptions, you can expect to be numb for approximately 2 to 3 hours after you leave the office. Want to learn what factors affect how long you will be numb?
To use or not to use Epinephrine
Perhaps the most important factor that affects how long you will be numb is the presence (or absence) of epinephrine.
Epinephrine acts to constrict the blood vessels where the anesthetic is injected. By constricting the blood vessels, you have less blood flow in and out of the numbed-up area, and the local anesthetic does not get carried away from the teeth and nerves as quickly. As a result, you remain numb longer and the numbing sensation is much more profound.
Occasionally, some of the epinephrine can end up in the bloodstream, leading your heart to beat more rapidly and some other symptoms. Some people then mistakenly assume they are allergic to epinephrine which is impossible. See this MythBuster post and a follow-up article on why this is not possible.
If you receive an injection with an anesthetic without epinephrine, on average, you will be numb for about 1 hour or slightly less after you leave. This assumes your dentist uses one of the two most common: 3% carbocaine/mepivicaine or Citanest Plain/4% Prilocaine.
Type of Anesthetic
Besides the presence of epinephrine, the type of local anesthetic also plays a role. First off, novocaine is not used anymore, so I personally have no idea how long novocaine would last.
Bupivacaine, which goes under the brand name Marcaine (at least in dentistry), can last a long time. In formulations without epinephrine, it can last 4 – 8 hours. When used in dentistry with epinephrine, it can last for 8 hours or more. In my experience, when I give bupivacaine with epinephrine for removing wisdom teeth, I tell patients that they can be numb until the next morning. Many of them report going to bed and still being numb.
Amount of Local Anesthetic Administered
This seems pretty obvious right? In general, the more you receive, the longer you’ll be numb. But remember that twice as much does not mean you will be numb twice as long. But you will be numb much longer.
For a straightforward filling, one shot (1.7 mL or cc) is typically sufficient, unless you are a patient who is difficult to get numb (see here and here on this phenomenon). For procedures that are a bit more involved, such as an extraction or root canal, dentists typically give more than one injection. The end result is you can often be numb for four or five hours afterwards.
Location, Location, Location!
Real estate is not the only area where this saying is relevant. The location – the area in the mouth where the local anesthetic is injected – plays a role in how long you will be numb.
For a lower back tooth to be worked upon, you nearly always needs a nerve block. Nerve blocks last longer than infiltrations – which is when the anesthetic is placed directly next to the tooth. In generalizing data from this paper, if you receive a nerve block, you will be numb approximately 25 to 30% longer than if you received an infiltration.
So what does all this mean? If you had a lower molar worked on, you were likely given a nerve block, and therefore you will likely be numb longer than if you received an injection for an upper front tooth.
This list is not exclusive. There are a lot of other factors – metabolism, genetics, hair color – but these four are the most common. I may do a follow-up post where I talk about the other factors. Comments are welcome.