Can Diet Pills Cause A False-Positive Opioid Test?

In out latest question and answer, the pharmacist discusses whether or not diet pills can cause a false-positive drug test for opioids or narcotics.


I'm just wondering if I'm being tested for narcotics, will taking a diet pill or pills such as truFix and truVision cause any kind of problems such as a false positive? I would just like to be sure that nothing along those lines will happen before I start taking diet pills.

Asked by Lizzie On Jul 26, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Jul 28, 2018

There have been very few studies looking at whether or not any dietary supplements can cause a false-positive test for narcotics, like opioids.

However, false-positives with urine based drug tests are fairly common, so it stands to reason that some supplements may cause a false positive. In addition, there are many documented drugs that are known to cause false positives for opioids. We have written extensively on the subject of false-positives for drug tests, which can be found here: What Causes False-Positives On Urine Drug Tests? Below are some highlights from that article:

  • Most common drug tests are simple urine based immunoassay tests, which work by using antibodies to react to specific drug compounds. When a specified drug is present in a test urine sample, an antibody binds to it, which produces a reaction. This reaction signifies a positive result.
  • Unfortunately, immunoassay urine tests have a significant problem with 'cross-reactivity', meaning that drugs that are NOT being tested for can react with the wrong antibody (i.e. 'cross react'). Whenever a reaction occurs, even if it is with the wrong drug, a positive result is recorded, although in this sense it would be a "false-positive".

With opioids specifically, which are narcotics, there are many drugs that are known to cause false-positives for them. They include:

  • Dextromethorphan (Delsym)
  • Diphenhydramine (Benadryl)
  • Doxylamine (one of the ingredients in NyQuil)
  • Quinolone antibiotics (e.g. Ciprofloxacin, levofloxacin)
  • Quinine
  • Rifampin
  • Verapamil
  • Clomipramine

Sources: Urine Drug Screening, Commonly Prescribed Medications And False-Positives, Common False-Positive Substances 

Although there are no documented false-positives with diet pills (or the ones you mentioned in your inquiry like truFix), there could simply be a lack of data regarding the ingredients contained in them and their likelihood of causing altered test results.

In addition, over the counter dietary supplements are notorious for sometimes containing mis-branded and adulterated ingredients. The FDA has a list of supplements that have included ingredients that are either not specified on the label or are prescription only ingredients.

Diet pills are probably more likely to cause false-positives for stimulants rather than narcotics, as numerous products are included on the FDA list that contain amphetamine like substances. Unfortunately, it is nearly impossible to guarantee that an over the counter supplement will not cause a false-positive reading due to the lack of data regarding many of the ingredients.

About the Pharmacist

Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Office: 716-389-3076

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