# Lidocaine showing up as false positive for cocaine? **URL:** \ **Category:** Factual Questions\ **Created:** [September 9, 2003, 7:20pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588 "2003-09-09T19:20:45Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![Bijou\_Drains](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bijou_drains/32/2814_2.png) [@Bijou\_Drains](https://boards.straightdope.com/u/Bijou_Drains)\ **Post date:** [August 9, 2015, 5:31pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/21 "2015-08-09T17:31:49Z")
FYI dentists stopped using novocaine decades ago. They all use lidocaine now. But they don’t bother to tell anyone what they use so everyone just assumes it’s novocaine. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [August 9, 2015, 6:11pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/22 "2015-08-09T18:11:56Z")
> [@markn\_1](#): > > The elimination half life of lidocaine is less than 2 hours. Even if you’d been injected with 1000 mg of lidocaine (an absurdly large amount), after 3 months (about 1000 half lives), the chances that there is a _single molecule_ of lidocaine left in your body is less than one in 10^90. Even if they were specifically testing for lidocaine rather than cocaine, there’s no way it would have been detected. > > –Mark He mentioned a hair test. Would it remain in your hair for weeks or months? Specifically, if lidocaine has such a short half-life, would any detectable amount ever get into your hair in the first place? ---
**Author:** ![rsat3acr](https://avatars.discourse-cdn.com/v4/letter/r/a3d4f5/32.png) [@rsat3acr](https://boards.straightdope.com/u/rsat3acr)\ **Post date:** [August 9, 2015, 7:32pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/23 "2015-08-09T19:32:19Z")
> [@Bijou\_Drains](#): > > FYI dentists stopped using novocaine decades ago. They all use lidocaine now. But they don’t bother to tell anyone what they use so everyone just assumes it’s novocaine. Most local anesthetic used by dentists in this country is now Septocaine(Articaine) IIRC something like 60%. In Europe I believe it is something like 90%. Lidocaine is number two. Septocaine if more lipidophillic, therefore works better than lidocaine. Haven’t been able to buy Novocaine by itself in a dental carpule sine before I started 29 years ago. Could buy it in combination with another anesthetic I think into the late 90’s. I think it is like so many other things. Everyone Xeroxes someting no mater what brand of dry process copier they use and blow their nose with a Kleenex no matter what tissue they really use. I think Novocaine as a local anesthetic is the same way. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [August 9, 2015, 7:35pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/24 "2015-08-09T19:35:52Z")
> [@Senegoid](#): > > He mentioned a hair test. Would it remain in your hair for weeks or months? Ah good catch. I know nothing about the biochemistry, but I would guess that if it did get into your hair it might stay there until the hair was cut, since no metabolism happens in the hair itself. But there would probably only be about a day or two during which it would be possible to get into the hair, after which the level in your blood falls to undetectable levels. Hair grows about a half millimeter a day. Whether a half millimeter of hair could contain detectable levels I don’t know. This would also assume he hadn’t gotten his hair cut in the 3 months between the injection and the test. And assuming that lidocaine can trigger a positive for cocaine in the first place, which seems to be denied by most sources. –Mark ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [August 9, 2015, 8:00pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/25 "2015-08-09T20:00:06Z")
Pro-tip: Always shave your head before starting a new job (or whenever the drug test is scheduled). Try to do this a few weeks ahead of time (if you know that far in advance) so it isn’t too obvious. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [August 9, 2015, 8:44pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/26 "2015-08-09T20:44:46Z")
> [@5-HT](#): > > … I wouldn’t take it as gospel, but I found a bunch of references to Tetracycline and Amoxacillin causing false positives. That seems unlikely – Tetracycline and Amoxacillin are from very different classes of antibiotics. I would expect that if this is true, it would be for most of the -cycline drugs, or most of the -cillin drugs, but not just one of each of those classes. Are you sure you haven’t mixed up the names of the drugs involved? ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [August 9, 2015, 9:10pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/27 "2015-08-09T21:10:13Z")
> [@markn\_1](#): > > The elimination half life of lidocaine is less than 2 hours. Everybody knows that. That’s about how long it takes after the dentist for your lips, gums, jaw & face to start tingling, throbbing, and otherwise letting you know that the anesthetic is starting to wear off. ---
**Author:** ![smithsb](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smithsb/32/12107_2.png) [@smithsb](https://boards.straightdope.com/u/smithsb)\ **Post date:** [August 9, 2015, 10:33pm UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/28 "2015-08-09T22:33:54Z")
😃 Hopefully the stuff is out of his system after TWELVE years. 😃 ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [August 10, 2015, 1:16am UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/29 "2015-08-10T01:16:07Z")
> [@smithsb](#): > > 😃 Hopefully the stuff is out of his system after TWELVE years. 😃 Depends if you cut your hair. ---
**Author:** ![foolsguinea](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/foolsguinea/32/14751_2.png) [@foolsguinea](https://boards.straightdope.com/u/foolsguinea)\ **Post date:** [August 10, 2015, 2:07am UTC](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588/30 "2015-08-10T02:07:06Z")
How long is your hair? [Previous page](https://boards.straightdope.com/t/lidocaine-showing-up-as-false-positive-for-cocaine/200588.md?page=1)