Brief Answer:
Information

Detailed Answer:
Hello XXXXXXX

I looked in multiple research articles on prescription medications that can cause false positives on drug screens, and gabapentin/Neurontin did not show up in the lists of any of these professional research studies. The false positive may be from one of the other medications you were taking or even one that they gave you while in the hospital IV that you were not aware of.

I have provided 2 research articles on the subject.

Here is a link to the first article (too long to print here). It won't show up as a blue link to click on, so you will need to copy and paste it into your address bar:
http://www.medscape.com/viewarticle/726897_2

The second is an abstract that lists all of the medications:

Am J Health Syst Pharm. 2010 Aug 15;67(16):1344-50. doi: 10.2146/ajhp090477.
Commonly prescribed medications and potential false-positive urine drug screens.

Brahm NC1, Yeager LL, Fox MD, Farmer KC, Palmer TA.
Abstract
PURPOSE:
The implications of potential false-positive urine drug screen (UDS) results for patients receiving commonly prescribed medications were evaluated.
SUMMARY:
A comprehensive literature review was conducted to identify false-positive UDSs associated with all clinic formulary medications, as well as common nonprescription medications. The references of each report describing a medication whose use was associated with false-positive UDS results were also reviewed. If a class effect was suspected, additional agents in the category were searched. A total of 25 reports of false-positive UDS results were identified. Categories of medications included antihistamines, antidepressants, antibiotics, analgesics, antipsychotics, and nonprescription agents. Reports of false-positive results were found for the following formulary and nonprescription medications: brompheniramine, bupropion, chlorpromazine, clomipramine, dextromethorphan, diphenhydramine, doxylamine, ibuprofen, naproxen, promethazine, quetiapine, quinolones (ofloxacin and gatifloxacin), ranitidine, sertraline, thioridazine, trazodone, venlafaxine, verapamil, and a nonprescription nasal inhaler. False-positive results for amphetamine and methamphetamine were the most commonly reported. False-positive results for methadone, opioids, phencyclidine, barbiturates, cannabinoids, and benzodiazepines were also reported in patients taking commonly used medications. The most commonly used tests to screen urine for drugs of abuse are immunoassays, even though false-positive results for drugs of abuse have been reported with a number of these rapid-screening products. Results from such tests should be confirmed using additional analytical methods, including gas chromatography-mass spectrometry.
CONCLUSION:
A number of routinely prescribed medications have been associated with triggering false-positive UDS results. Verification of the test results with a different screening test or additional analytical tests should be performed to avoid adverse consequences for the patients.
PMID: 0000 DOI: 10.2146/ajhp090477
[Indexed for MEDLINE]

I recommend that you request the next step of a more sophisticated drug assay that can show you are not taking methadone.

I hope this information helps.

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Brief Answer:
Information

Detailed Answer:
It is documented that diphenyhdramine and ibuprofen can both give a false positive for methadone.

I can't find specific information on Norco causing this, but being that it is in the opiate family, it might. I just can't find literature for this specifically causing a false positive for methadone.

Given the medications you regularly take, it seems that a more advanced test, such as GC-MS or others, would be needed. Otherwise they won't be able to distinguish what you are taking from other things.

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