From a doctor that had been repped highly and labeled an expert on the site:
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Naltrexone is technically an opioid, that is a synthetic opiate. It does not produce the same metabolites as the opiate class and it is these metabolites that are being tested for. Therefore, this drug should not be detected unless specifically tested for. The 5-panel will not pick it up at 2000 or anywhere near that cut-off. Tests for detection must usually be "no threshhold" to detect. You should be entirely safe. But it is technically detectable.
It seems to be the concensus from googling. But here's a quote from Ann2435:
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I do want to give one warning. To the best of my understanding, naltrexone is metabolized as a narcotic. That means that if you are subject to a urine test for drugs, say for employment, a urine drug screening may show positive results for opioids. I don't believe the test can discriminate among the different classes. Positive results for heroin or the codeine in your cough syrup or naltrexone, would all be the same. Now, if you have a prescription for naltrexone or codeine or hydrocodone or a drug in this family you will be safe.