What Drugs Cause Fentanyl False-Positives?

While urine drug screenings are notorious for their high rate of false-positives (i.e. testing positive for a drug that isn't actually present in a sample), false-positive results for fentanyl are less common.

Fentanyl is a fully synthetic opioid and is sometimes even described as a 'designer' opioid. It is structurally distinct enough from other opioids that it isn't reliably detected on standard opioid urinalysis tests, and, if testing for it is necessary, specific tests need to be ordered.

One study, Laboratory Testing for Prescription Opioids, discusses this about fentanyl:

Other prescription opioid drugs such as fentanyl and buprenorphine are sufficiently distinct in structure compared to morphine that these drugs show essentially no reactivity in commonly marketed morphine-specific opiate immunoassays. Detection of these opioid drugs therefore requires entirely separate immunoassays that are specific for these compounds or methods capable of their detection and specific identification such as liquid chromatography–tandem mass spectrometry.

Even though a false-positive result for fentanyl isn't common, it has still been reported to happen, and this is simply due to how urinalysis drug tests work.

Essentially, urine drug tests use antibodies to react to a specific drug compound. When the drug compound being tested for is present, the antibody binds to it and produces a reaction. This is recorded as a positive result.

Unfortunately, antibodies can sometimes bind to the wrong substance and react. This is what causes a 'false-positive' result.

Most commonly, when an antibody binds to the wrong substance, the molecular structure if similar to what is being tested for. 

When it comes to fentanyl, as mentioned above, it is fairly unique and therefore isn't often associated with false-positive reactions.

While false-positive tests for fentanyl don't happen often, several case studies have been published identifying drugs that may have been responsible for false-positive results.

The four drugs with the most evidence for causing false-positives for fentanyl are:

• Trazodone
• Risperdal (risperidone)
• Invega (paliperidone)
• Fanapt (iloperidone)

Risperidone and related compounds (e.g. paliperidone, iloperidone) have been extensively reported to be responsible for causing false-positive drug tests for fentanyl.

In fact, risperidone is listed as a potential drug to be aware of that may interfere with fentanyl urine tests in educational bulletins and product information for drug tests.

If you believe you have mistakenly received a false-positive result for fentanyl, you should request a more accurate confirmatory test.

Urine drug tests should ideally be utilized as an initial screening only since they are subject to false-positives.

Other, more accurate tests, like GS-MS, can detect specific compounds in a given sample, and don't rely on antibody reactions for results.