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Dr. Andrew Rynne

Family Physician

Exp 50 years

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Can Amoxicillin cause a false positive in a drug test result?

Answered by
Dr. Bonnie Berger-Durnbaugh

General & Family Physician

Practicing since :1991

Answered : 3104 Questions

Posted on Fri, 3 Mar 2017 in Medicines and Side Effects
Question: Can amoxicillin cause a false positive drug result
Answered by Dr. Bonnie Berger-Durnbaugh 1 hour later
Brief Answer:

Detailed Answer:

The medical literature is somewhat contradictory on this subject, but multiple sources suggest amoxicillin can give a false positive for cocaine.

If this happened to you, I recommend requesting a follow up test called GC-MS which is more accurate than standard urine drug screens.

Also, having your doctor document that you were on amoxicillin when the drug test was done can be helpful.

Here is an article on the subject that might be helpful also:

I hope this helps.
Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar
Follow up: Dr. Bonnie Berger-Durnbaugh 11 hours later
The test company didn't ask if I was on any medication at the time . I have also heard that me being a diabetic can cause the same results is this accurate ?
Answered by Dr. Bonnie Berger-Durnbaugh 6 hours later
Brief Answer:

Detailed Answer:
Yes, kidney problems, liver disease, and diabetes can all cause false positives on urine drug screens.

If this test was done for employment purposes, consider contacting the health dept. or human resources at the company you are or are seeking employment with and ask about if you can get them a letter from your doctor explaining your medical diagnoses and medications.
Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar
Follow up: Dr. Bonnie Berger-Durnbaugh 31 hours later
Do you have any available liderature that supports this because I would really like to have this for support on my case seeing as your the only doctor that gives me an answer on this subject
Answered by Dr. Bonnie Berger-Durnbaugh 2 hours later
Brief Answer:
Amoxicillin and false positives

Detailed Answer:
I'm not sure if you want literature on diabetes or amoxicillin regarding false positives.

Regarding amoxicillin, see Table 3 in the article below:

The article above comes from University of Illinois College of Pharmacy.

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar
Follow up: Dr. Bonnie Berger-Durnbaugh 21 hours later
Answered by Dr. Bonnie Berger-Durnbaugh 31 minutes later
Brief Answer:
Hopefully this will work.

Detailed Answer:
Hi - so hopefully this will work. I copy and pasted it as text here. Scroll down to find Table 3.

University of Illinois-Chicago, College of Pharmacy, Feb. 2011:

What drugs are likely to interfere with urine drug screens?

Urine drug screens (UDS) are a frequent practice used to detect common drugs of abuse. On-site drug screening is performed for a variety of medical, professional, and legal reasons. A few scenarios in which screening may be done are listed below:1

Reasons for urine drug screening1

Suspicion of drug abuse (e.g., unexplained negligence/impairment/behavior)
Random testing outlined in employment contract
Military service
Sports participation
Legal/criminal (e.g., postaccident, parole)
Drug-therapy compliance monitoring
Drug abuse rehabilitation monitoring
Postmortem investigation
Screening Methods

Urine drug screens are generally performed using either immunoassays or gas chromatography-mass spectrometry (GC-MS).2 Immunoassay UDS contain specific antibodies against common drugs of abuse and their metabolites. The immunoassay is the most commonly used UDS because it is inexpensive and rapid. Five different immunoassays are available: cloned enzyme donor immunoassay, enzyme-multiplied immunoassay (EMIT), fluorescence polarization immunoassay (FPIA), and immunoturbidimetic assay radioimmunoassay (RIA). The substances most commonly tested by a typical immunoassay include amphetamines, cannabinoid metabolites, cocaine metabolites, opiate metabolites, and phencyclidine (PCP). Expanded immunoassays are available to test for tricyclic antidepressants, barbiturates, methadone, alcohol, and benzodiazepines and may be beneficial when use of these substances is suspected. One major problem with immunoassays is a false-positive result. Therefore, a more specific confirmatory test, such as GC-MS, is needed to confirm a positive finding with an immunoassay. GC-MS is more accurate than an immunoassay, but it is more expensive and time consuming.1 GC-MS breaks down drug molecules into ionized fragments and identifies substances based on mass-to-charge ratio using a mass spectrometer.

Although blood, hair, nails, or saliva can be used, most screening is done using urine samples.2 Ease of collection, higher drug concentrations, and longer durations of detection are primary reasons for using urine samples for drug screening. Table 1 lists common drugs of abuse and their duration of detection in the urine.

Table 1. Length of time drugs of abuse can be detected in the urine.1,2

Drug/drug class     Detection time          Drug/drug class     Detection time
Alcohol     7 to 12 hours          Marijuana     
Amphetamine     48 hours          Single use     3 days
Methamphetamine     48 hours          Moderate use (4x/week)     5 to 7 days
Barbiturates               Daily use     10 to 15 days
Short-acting     24 hours          Long-term heavy smoker     >30 days
Long-acting     3 weeks          Opiates     
Benzodiazepine               Codeine     48 hours
Short-acting     3 days          Heroin (morphine)     2 to 4 days
Long-acting     30 days          Hydromorphone     2 to 4 days
Cocaine metabolites     2 to 4 days          Methadone     3 days
Morphine     48 to 72 hours
Oxycodone     2 to 4 days
Propoxyphene     6 to 48 hours
Phencyclidine     8 days
Collection Methods and Criteria

Proper urine collection methods must be used to avoid false-negative results. Urine should be collected in a tamper-evident container under supervision if necessary.1 Criteria for legitimate urine samples include:

A volume of 30 mL or more
Temperature between 32▫C and 38▫C
pH of 4.5 to 8.5
Nitrates <500 mcg/mL
Specific gravity >1.002 and <1.020
Creatinine >20 mg/dL
Urine samples with characteristics outside of these ranges, or with a cloudy or dark appearance, may be adulterated in some manner, either diluted or substituted.

Urine Drug Test Results

The United States Department of Health and Human Services (DHHS) sets the threshold for drug concentrations for detection by UDS.2 Drug concentrations in the urine below this level are reported as negative. Table 2 contains a list of these values. However, despite standardization, inaccurate results can occur.

Table 2. Standard threshold levels for screening and confirmatory tests 1,3

Drug/drug class     Immunoassay screena (ng/mL)     GC/MS confirmationa (ng/mL)
Amphetamine and methamphetamine     1000     500
Barbiturates     300     200
Benzodiazepines     300     200
Cocaine metabolite (benzoylecgonine)     300     150
Marijuana metabolites (delta-9-tetrahydrocannabinol-9-carboxylic acid)     50     15
Methadone     300     200
Opiates (codeine and morphine)     2,000     2,000
Phencyclidine     25     25
Propoxyphene     300     200
a Standard cutoff levels; alternate cutoff levels may be available.

Abbreviation: GC-MS, Gas chromatography-mass spectrometry.

False-Negative Results

False negatives are uncommon but can occur as a result of low drug concentrations in the urine, tampering, and in other situations. Possible reasons for false-negative results include: 1,2

Dilute urine (excess fluid intake, diuretic use, pediatric sample)
Infrequent drug use
Prolonged time since last use
Recent ingestion
Insufficient quantity ingested
Metabolic factors
Inappropriate test used
Elevated urine lactate
Tetrahydrozoline (eye drops)
Lemon juice
Drain cleaner
Table salt
Various chemicals (glutaraldehyde, sodium or potassium nitrate, pyridinium chlorochromate, and peroxide/peroxidase)
Understanding the UDS and ordering the appropriate test can prevent false-negative results. Results from an immunoassay or a GC-MS can be deceiving, as these tests may not be able to detect every drug in a particular drug class.2 This particularly pertains to the opiate and amphetamine/methamphetamine immunoassays. For example, a test for opiates will detect morphine and drugs that are metabolized to morphine, such as codeine and heroin. Heroin itself can only be detected for up to 8 hours after use. After 8 hours, only the morphine metabolite of heroin will be detected in the urine by immunoassay or by GC-MS. Other opiates such as fentanyl, oxycodone, methadone, hydrocodone, buprenorphine, and tramadol will not be detected and require an expanded immunoassay panel for detection.1 The amphetamine/ methamphetamine immunoassay can detect racemic compounds (dextroamphetamine, methamphetamine) and illicit analogues (methylenedioxyethylamphetamine, methylenedioxyamphetamine, and methylenedioxymethylamphetamine [MDMA]). This assay, however, has a low sensitivity for MDMA and a more specific test should be performed if MDMA is suspected.

False-Positive Results

Although immunoassays are very sensitive to the presence of drugs and drug metabolites, specificity and accuracy varies depending on the assay used and the substance for detection.2 This limitation may result in false-positives from substances cross-reacting with the immunoassay. Positive results seen on immunoassay need to be confirmed using the more accurate GC-MS, the forensic standard. The DHHS detection limits reduce false-positive results, but do not eliminate them. In 1998, the cut-off for opiates was raised from 300 ng/mL to 2000 ng/mL to avoid false positives from XXXXXXX seed ingestion. However, these more stringent requirements can lead to false-negatives and many laboratories continue to use the lower value for detection. For example, detectable levels of cannabinoids after ingestion of hemp-containing foods with immunoassay have been reported. Levels of cannabinoids in these samples, however, were not detectable with GC-MS. Passive marijuana or cocaine smoke inhalation has never been documented to achieve detectable urine concentrations in adults, however, passive cocaine smoke inhalation has achieved detectable levels in pediatric cases.

GC-MS is very accurate; however, it is not without problems in drug detection.2 As mentioned earlier, heroin and hydrocodone are metabolized into morphine and hydromorphone respectively, and GC-MS may identify the metabolites rather than the parent compound. Selegiline is metabolized to l-amphetamine and l-methamphetamine, isomers without central nervous system stimulation. Neither immunoassay nor GC-MS can differentiate between the l and d isomers and a positive result for amphetamines will be found; an alternative test, chiral chromatography, may be needed.

Many prescription and nonprescription substances have been reported to cross-react with immunoassays and cause false-positives.2 Most have only been documented in case reports. Table 3 lists substances reported to cause false-positive results using immunoassay. This list may not include all potential substances. The frequency of false-positives varies, depending on the specificity of immunoassay used and the substance under detection. Immunoassay results for cannabinoid and cocaine metabolites are associated with very few false-positives while immunoassay results for amphetamines and opiates are associated with a higher number of false-positives.1

Table 3. Substances that may cause false-positives on immunoassay urine drug screens.1-7

Drug/drug class     Interfering drug          Drug/drug class     Interfering drug
Amphetamine and     Amantadine          Cocaine     Amoxicillin
methamphetamine     Brompheniramine               Coca leaf teas
Bupropion               Tonic water
Chlorpromazine          Methadone     Chlorpromazine
Desipramine               Clomipramine
Desoxyephedrine               Diphenhydramine
Ephedrine               Doxylamine
Fluoxetine               Ibuprofen
Isometheptene               Quetiapine
Isoxsuprine               Thioridazine
Labetalol               Verapamil
Phentermine          Opiates     Dextromethorphan
Phenylephrine               Diphenhydramine
Phenylpropanolamine               Fluoroquinolonesa
Promethazine           XXXXXXX seeds and oil
Pseudoephedrine               Rifampin
Ranitidine               Quinine
Selegiline          Phencyclidine     Dextroamphetamine
Thioridazine               Dextromethorphan
Trazodone               Diphenhydramine
Trimethobenzamide               Doxylamine
Trimipramine               Ibuprofen
Vicks inhalerb               Imipramine
Barbiturates     Ibuprofen               Ketamine
Naproxen               Meperidine
Benzodiazepines     Oxaprozin               Thioridazine
Sertraline               Tramadol
Cannabinoids     Dronabinol               Venlafaxine
Efavirenz          Tricyclic     Carbamazepine
Hemp-containing foods          antidepressants     Cyclobenzaprine
Ibuprofen               Cyproheptadine
Ketoprofen               Diphenhydramine
Naproxen               Hydroxyzine
Piroxicam               Quetiapine
Promethazine          Lyseric acid     Amitriptyline
Proton pump inhibitorsc          diethylamine (LSD)     Dicyclomine
Sulindac               Ergotamine
Tolmetin               Promethazine
a Ciprofloxacin, levofloxacin, and ofloxacin.

b Vicks inhaler due to l-methamphetamine content interfered with older immunoassays; interference has not been seen with new enzyme multiplied immunoassay tests (EMIT).

c Pantoprazole.


The strengths and limitations of UDS need to be fully understood in order to perform the correct screen and also to correctly interpret the results. All positive results on immunoassay are presumptive until confirmed using GC-MS. An extensive medication history including prescription, nonprescription, and herbal medications should be obtained from the patient. Medication histories are important in order to anticipate false-positives as well as differentiate between drugs used for legitimate medical purposes and drugs of abuse.


1. Standridge JB, XXXXXXX SM, Zotos AP. Urine drug screen: a valuable office procedure. Am Fam Physician. 2010;81(5):635-640.

2. Moeller KE, XXXXXXX KC, Kissack JC. Urine drug screening: practical guide for clinicians. Mayo Clin Proc. 2008;83(1):66-76.

3. Quest Diagnostics. Standard urine testing for drug and alcohol abuse.

Accessed Nov 11, 2010.

4. XXXXXXX EC, Zebelman A, Goodwin C. What common substances can cause false positives on urine drug screens for drugs of abuse? J Fam Pract. 2006;55(10):893-894, 897.

5. Brahm NC, Yeager LL, Fox MD, Farmer KC, Palmer TA. Commonly prescribed medications and potential false-positive urine drug screens. Am J Health-Syst Pharm. 2010;67(16):1344-1350.

6. Holtorf K. Ur-ine Trouble. XXXXXXX AZ: Vandalay Press; 1998.

7. Woelfel JA. Drug abuse urine tests: false-positive results. Pharmacist's Letter/Prescriber's Letter. 2005;21(3):210314.

Prepared by: XXXXXXX XXXXXXX PharmD

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar
Follow up: Dr. Bonnie Berger-Durnbaugh 15 hours later
Would my family doctor know I had a procedure done if he didn't send me to it?
Answered by Dr. Bonnie Berger-Durnbaugh 2 hours later
Brief Answer:

Detailed Answer:
By procedure, do you mean the urine drug screen? If it was done for employment purposes by a future or current employer, then no, your own doctor would probably not know about it.
Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar
Follow up: Dr. Bonnie Berger-Durnbaugh 3 minutes later
Sorry I wasn't specific I had a procedure done with my nose a the hospital because I Broke it would my family doctor know this
Answered by Dr. Bonnie Berger-Durnbaugh 41 minutes later
Brief Answer:

Detailed Answer:
Most likely the notes that the doctor at the hospital wrote about the procedure would have been shared with your family doctor, or at least your family doctor would have access to them. I am not sure how the Canadian system works, but in the US, usually hospital notes will be sent to the primary doctor.
Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar

The User accepted the expert's answer

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