Martin S. Spiller, DMD, Family Dentistry in Townsend, MA

Martin S. Spiller, DMD, Family Dentistry in Townsend, MA

Can Dental Anesthesia cause False Positive Drug Tests?





This is the last of seven pages which constitute a course in local anesthetics.  Each page stands on its own, however for a thorough understanding of dental local anesthetics the reader is advised to read the pages in order.

<==Allergies to local anesthetic


Can any local anesthetics cause false positive drug tests?

Dental syringeLocal anesthetic injections before a drug test do NOT cause false positive results for cocaine in urine tests!    Urine testing for cocaine involves an immunoassay using antibodies against the major metabolites of cocaine, ecgonine methyl ester and benzoylecgonine.  These chemicals are  unique to the metabolism of cocaine and will not appear in the urine if the patient has been injected with lidocaine or other dental anesthetic.

In order to manufacture the lab test kit, the manufacturer injects ecgonine methyl ester and/or benzoylecgonine into the veins of a large laboratory animal, such as a horse or a sheep.  The animal's immune system then naturally makes antibodies against them, and these antibodies circulate in the animal's blood.  The animal's blood is then drawn, and the antibodies are extracted from the plasma.  These antibodies are then refined and used in the testing kit to bind to the ecgonine metabolites in the testee's urine.  If the antibodies bind to their target metabolites, then the solution turns a specific color indicating a positive test.  According to expert opinion, cross reactivity between urine immunoassay screens for these metabolites and substances other than cocaine are nearly nonexistent! 

Other testing issues

Even though mainstream opinion confirms the infallibility of the immunoassay for cocaine, the same cannot be said for other drugs.  Immunoassays are not the best way to test for metabolites or the drugs themselves, even though they are the least expensive and most readily available type of urine test on the market.  The natural function of antibodies is to recognize proteins characteristic of viruses and bacteria.  While antibodies are good at recognizing large protein molecules, they are not as specific in recognizing especially small molecules.  Drug metabolites, as well as the drugs themselves have small molecular structures.  The argument against urine immunoassay testing suggests that the drug test kit manufacturer generally knows the rates of false positive results associated with any given batch of their test kits, although they are not often very forthcoming about this.  The rate varies depending on the specificity of the batch of antibody that accompanies the kit.

The ramifications of this are a bit complex.  The only definitive way to confirm that a positive urine test for cocaine (or any other drug) is actually due to recent exposure to the actual drug, and not to a substance which may produce a false positive result, is to perform another test called gas chromatography-mass spectrometry (GCMS) on the same sample.  If you have recently used cocaine or any other controlled drug, this test is irrefutable (provided that the test was run by a competent lab), but if your urine originally tested as a false positive due to the lack of specificity of the testing technique, or for any other reason, a GCMS will clear your name. 

Unfortunately, these tests are expensive, and a prospective employer is under no obligation to retest your urine, since he can refuse to hire you for any reason except those based on sexual or racial discrimination. 

On the other hand, if you already hold a job which you may lose due to a random drug test, or if there is a chance that the test might result in legal consequences beyond losing your job, then the employer or testing authority should run the more expensive GCMS test on the original sample to confirm your drug status.  If the employer refuses, or if he did not retain the original sample, then you may have an actionable legal case against the employer.

Length of time drug abuse can be detected in urine

Alcohol  7-12 h
Amphetamine  48 h
Methamphetamine 48 h
     Short-acting (e.g., pentobarbital) 24 h
     Long-acting (e.g., phenobarbital) 3 wk
     Short-acting (e.g., lorazepam) 3 d
     long-acting (e.g., diazepam)  30 d
Cocaine metabolites 2-4 d
     Single use 3 d

     Moderate use (4 times/wk)

5-7 d
     daily use 10-15 d
     Long-term heavy smoker  >30 d
     Codeine 48 h
     Heroin (morphine) 48 h
     Hydromorphone 2-4 d
     Methadone 3 d
     Morphine 48-72 h


2-4 d
     Propoxyphene 6-48 h
     Phencyclidine 8 d

List of substances that can cause false positive urine immunoassays

There are lots of other problems with drug testing, especially as it relates to employee screening in today's large corporate structures.  False positives may be the result of a range of problems from cross reaction with over-the-counter and prescription medications, to systemic conditions such as diabetes, or kidney and liver disease. Plain sloppiness on the part of the testing facility may also be a factor.  

The following is a list of illegal drugs, each followed by a number of commonly used and legally available substances (many of them prescription drugs) that may cross-react and cause a false positive drug test.

Note that the use of prescription (DEA schedule II, III, IV or V)  drugs without a doctor's prescription is illegal and are grounds for the legal consequences stemming from a positive drug test.  (Schedule I includes drugs such as heroine, LSD, ecstasy, designer and experimental drugs.   None of these are legal to prescribe to the general public.)

Substance tested via immunoassay

Potential agents causing false-positive result

Most common brand name, or use

Alcohol Short-chain alcohols (e.g., isopropyl or Methyl alcohol) Rubbing alcohol, Wood alcohol
Amphetamines Amantadine Symmetrel--Treats Asian flu (Anti-viral) and Parkinson's
  Benzphetamine Didrex--diet drug
  Bupropion Wellbutrin--antidepressant, also used to help stop smoking
  Chlorpromazine Thorazine--phenothiazine used to treat schizophrenia
  Clobenzorex Asenlix, Finedal, or Rexigen "greenies" treats Parkinson's
  Deprenyl Carbex, Eldepryl--Treats Parkinson's
  Desipramine Norpramin--Tricyclic antidepressant, also for sleep disorders
  Dextroamphetamine Dexadrine (an anphetamine)
  Ephedrine Decongestant, used in many brands of  cold medicine
  Fenproporex Diet pill--Component of FenFen
  l-methamphetamine Vicks Nasal Inhaler--Can cause false positive for methamphetamine
  Isometheptene Used to treat migraines--component of Amadrine and Midrin
  Isoxsuprine Vasodilan--Vasodilator used to treat dementia, Raynaud's and poor circulation
  Labetalol Normodyne, Trandate--Beta Blocker--Blood pressure med
  MDMA Ecstasy--illegal drug
  Methylphenidate Concerta, Ritalin--Used for ADHD
  Phentermine Diet pill--Component of FenFen
  Phenylephrine,Phenylpropanolamine, Promethazine, Pseudoephedrine Decongests  used in nasal sprays and cold medicines
  Ranitidine Zantac, treats acid reflux
  Ritodrine Yutopar--Smooth muscle relaxant used to treat premature labor
  Selegiline Eldepryl--MAO inhibitor used to treat Parkinson's
  Thioridazine Mellaril--used to treat schizophrenia
  Trazodone Antidepressant--treats sleep disorders
  Trimethobenzamide Tigan, used to treat nausea
  Trimipramine Surmontil--Tricyclic antidepressant
Benzodiazepines (like valium and Ativan) Oxaprozin Daypro--An NSAID similar to ibuprophen
  Sertraline Zoloft--Treats depression and OCD
Cannabinoids (marijuana or THC) dronabinol Marinol-Manmade Cannabis used to treat Nausea in cancer patients
  Efavirenz Sustiva--For treating HIV
  Hemp-containing foods Mostly sold in health food shops
  NSAIDs Advil, Motrin, Aleve--These OTC drugs treat pain and inflammation
  Proton pump inhibitors Zantac, Pepsid, Prilosec, Prevacid, Protonix--Used to treat Acid Reflux disease
  Tolmetin Another NSAID not often prescribed
  Efavirenz Antiretroviral (HIV)
  Sulindac Another NSAID


Used to treat nausea
Cocaine Coca leaf tea Illegal in USA
  Amocxicillin Comon antibiotic. There is little clinical evidence showing amoxicillin causes false positive drug tests.
  Tonic water This is NOT seltzer. Tonic water is a sweetened carbonated beverage containing quinine.
  Topical anesthetics containing cocaine Cocaine is no longer used in dentistry, but is still used in eye drops in ophthalmology
Opioids, opiates, and heroin Dextromethorphan Cough suppressant found in Robitussin DM, Mucinex DM
  Diphenhydramine Benedryl--An antihistamine found in sleep, allergy and cold meds
  Opiates (codeine,hydromorphone
hydrocodone, morphine
and Heroin, Oxycontin)
These are all opioids and are illegal without a prescription.
  Poppy seeds Found in MANY food items--serious problem in urine immunoassay drug tests
  Quinine An Antimalial agent
  Quinolones A class of antibiotics including Cipro and Levaquin
  Rifampin Rifadin--An antibiotic used for tuberculosis
  Verapamil and its metabolites Isoptin, Verelan--Calcium Channel Blocker used to treat hypertension
Phencyclidine Dextromethorphan Cough suppressant-- Robitussin DM, Mucinex DM
  Diphenhydramine Benedryl--An antihistamine found in sleep and cold meds
  Doxylamine Aldex, Equate sleep aid--Antihistamine similar to Benedryl used as sleep aid
  Ibuprofen Motrin, Advil
  Imipramine Tofranil--Tricyclic antidepressant used to treat depression and sleep disorders
  Ketamine Surgical anesthetic
  Meperidine Demerol--an opiate similar to morphine
  Mesoridazine Serentil--Similar to thorazine, used to treat schizophrenia
  Thioridazine Mellaril--Similar to thorizine used to treat schizoprhenia
  Tramadol A low grade opioid for pain relief available OTC in some countries
  Venlafaxine, O-desmethylvenlafaxine Effexor--Antidepressant for major depression
Tricyclic antidepressants Carbamazepine Tegretol--An anticonvulsant used to treat epilepsy and atypical pain disorders
  Cyclobenzaprine Flexeril--A skeletal muscle relaxant
  Cyproheptadine Periactin--an antihistamine used to treat severe allergies
  Diphenhydramine Benedryl--An antihistamine used to treat allergies and as a sleep aid
  Hydroxyzine Vistaril, Atarax--Antihistamine--treats itches and nausea
Barbiturates Ibuprophen and Naproxin Motrin, Advil, Aleve. Over the counter pain meds
LSD Amitriptyline Tricyclic antidepressent
  Dicyclomine Bentyl, Treats irritable bowel syndrome, or spastic colon and colitis
  Ergotamine Migraine med
  Promethazine Phenergan, Major antihistamine used as sedative and for motion sickness

Imitrex, Migraine med

Drug testing: Your rights versus the employer

Employers are private individuals under the law.  They have a right to deny you employment for any reason (provided the reason is not based on racial or sexual discrimination).  This means that an employer's rights extend to demanding that a prospective employee agree not only to submit to a drug test before being hired, but also to signing an agreement that the employer may demand that the employee, once hired, must submit to drug tests on demand in the future.  

For their part, employers have been forced by modern litigation practices to take a hard line against any employee who uses drugs.  The mere presence of an employee who can be shown to be abusing illegal substances places that employer at huge risk of having to pay gigantic awards to a plaintiff and his lawyer.  If you are unfairly targeted for termination of employment due to drug testing, you have become a victim of the perversions of American tort law.

Much is made of the citizens' fourth amendment right to freedom from unreasonable search and seizure of person or property.  Unfortunately, this right applies only to federal or state cases in which a search may result in legal sanctions such as prison time or fines paid to the state.  Private parties such as an employee and an employer who have entered into a legal agreement requiring a search (the drug test) or a subsequent seizure (your job) based on the search are not covered by the fourth amendment.

While an employee drug test may result in the termination of the employee, and a prospective employee drug test (or refusal to submit to drug testing) may prevent a subject from being hired, these tests seldom result in state or federal charges being brought and therefore the subject's legal rights in these matters is fairly limited.  

What can you do if you feel that you were unfairly labeled a drug user due to a false positive drug test?

The majority of drug testing kits on the market are immunoassay tests (described at the top of this page). While false positive immunoassay tests are very rarely wrong, the cheaper kits have been known to produce false positives, as demonstrated in the table above. The only way to legally contest a false positive immunoassay test is to get a lawyer who is knowledgeable about this issue and then demand that a gas chromatography-mass spectrometry test be used to confirm the presence of the actual drug metabolites in the original urine sample.  A good lawyer should be able to argue for the re-testing of the urine, or, if the sample no longer exists argue that without advanced testing, the immunoassay results are inadmissible.  This is NOT something that a letter from me, or any other authority can do.  Only a good lawyer arguing in a court of law can accomplish this! Even if you beat the rap, your employer (and the judge) may require that you take random drug tests in the future, so even this method will probably not protect a person who habitually uses the drug.

Q. But I have a prescription for medical marijuana to treat chronic pain.  I live in a state in which such prescriptions are legal.  Can an employer legally fire me if I test positive for marijuana in my urine or blood?

A. YES, the employer can still fire you, even if you have a legal prescription for medical marijuana, or for any other drug that is illegal under federal law.  Even if state law makes a drug de facto legal, federal law trumps state law, and marijuana is still illegal according to federal statutes.  On another level, an employer may choose to terminate any employee if the employer suspects that the employee might be impaired on the job. 

Q. I live in Colorado where it is legal to buy and use marijuana for recreational purposes. can my employer still fire me?

A. Yes. The Colorado Supreme Court has ruled that businesses can fire employees who use marijuana during their off-time, including those with a legal prescription for medical pot.

Click here to be directed to a fairly good page explaining the ins and outs of drug testing


Do NOT ask me to write notes to your employer regarding your innocence of illegal drug use due to false positive drug testing.  I am not an expert in either drug law or the evolving ability of drug tests to winnow out false positive results.  You will need to consult a lawyer who specializes in defending clients against wrongful termination

Dental syringe




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