What Causes False Positives on Drug Tests?

By Elizabeth Brico 04/08/19

Most instant drug tests are notorious for picking up false positives from common medications like antihistamines, antidepressants, antipsychotics, antibiotics, and analgesics. Poppy seeds can give a "true" positive.

Alarmed woman at desk, receiving false positive drug test result from boss
If you’re being tested on-site for a job, it’s entirely possible that your employer does not have a system in place for sending your sample to be examined in a lab.

I had a routine during my pregnancy with my elder daughter. Each morning I woke up as late as possible—which never felt late enough—took a quick shower, and waddled over to my bus stop. There, while waiting for the bus, my senses sharpened in the thin, crisp mountain air and the yellow morning sunlight stretching its way across Boulder, Colorado. Sometimes I snoozed a little more on the bus—I’ve always been a sucker for vehicular motion. On less sleepy days, I watched out the window for prairie dogs bopping across the acres and acres of lush green land.

I was riding into town for Naropa University, where I was attending grad school in the footsteps of Allen Ginsberg, Anne Waldman, and William Burroughs. But every day I turned into downtown several hours early for my classes. It wasn’t by choice, but because I was taking methadone to treat my addiction to heroin.

Being new to the program meant I hadn’t yet earned take-home doses, so I had to ride in every day before the clinic closed and drink down my syrupy pink dose in front of a nurse. It was annoying, but I discovered a small comfort: my bus dropped me off next to a small, vegan-friendly grocery store called Sprouts. So before I dosed, I would stop in and treat myself to piece of sticky-sweet, lemon poppy seed cake. It would not take long for me to discover the weird, unexpected consequence of my treat.

How to Get a False Positive for Opioids

“Your UA was positive,” the nurse said, lips pursed, about two months into the program. I wasn’t showing yet but all the staff knew about my pregnancy.

“For what?” I asked.


I laughed. “Well I’m on methadone.” At the time, I didn’t know clinics could differentiate between synthetic and non-synthetic opioids.

“No, not the methadone.”

Now I was pissed. I hadn’t used—not since enrolling in the program. Earning a take-home would depend on my compliance with the program, which meant testing negative every time they demanded I pee for them. Worse, a positive drug test during pregnancy could mean a child services investigation down the line.

“I didn’t relapse,” I insisted. The nurse just stared at me. Then I remembered that urban myth I’d heard—that eating poppy seeds could trigger an opiate positive on a drug test. “I’ve been eating poppy seed cake,” I told the nurse.

“You’d have to eat a whole lot of poppy seeds for that to happen,” she said.

But I insisted that the positive was wrong. Finally, she relented and agreed to send my sample for confirmatory testing. A few days later, she reported that the levels of morphine in my urine sample suggested it had, in fact, come from a food source. Turns out, poppy seed positives are not an urban legend at all—in fact, they are even recognized by the U.S. government, which actually raised the opioid detection cutoffs to avoid these types of false positives for military personnel and other government employees.

The Problem with Poppy Seeds

Poppy seeds trigger a positive for morphine. Opium and its derivatives—which means any naturally occurring opioid—come from papaver somniferum, a type of poppy plant. It is grown commercially for the development of pharmaceutical drugs and for the harvesting of food-grade poppy seeds. But because of their origin, these seeds can contain tiny amounts of opioid alkaloids, which metabolize similarly to morphine or codeine. It’s not enough to produce a euphoric effect—but it can be enough, depending on how much is consumed, to trigger a positive on a drug test. And that positive is, in fact, a “true positive,” at least in the sense that your body produced that metabolite.

Poppy seeds will trigger a positive for opioids on a general panel, or for morphine and sometimes codeine on a more detailed test. The problem here is that other opiates—including heroin—will also trigger a morphine positive. Heroin has its own unique metabolite, 6-monoacetylmorphine, but that will only show up for about 24 hours, whereas morphine from heroin use can show for up to a week.

When my nurse said the test confirmed my positive was the result of poppy seeds, she probably meant the levels were too low to show up in the confirmatory test. The truth is that there is no way to definitively link a morphine positive to poppy seeds, leaving the decision ultimately up to clinical judgment.

“They do try to correct for this by establishing cutoff limits,” says Ryan Marino, an emergency medicine physician and toxicologist with the University of Pittsburgh Department of Medicine. “So the person who is running the test might see the positive but it’s below the threshold, so it gets reported as negative.”

In the late ‘90s, the Substance Abuse and Mental Health Services Administration (SAMHSA) changed the detection cutoff for morphine from 300 ng/mL to 2000 ng/mL in an attempt to prevent federal employees from losing their jobs over a bagel topping. While a bagel probably won’t trigger detection at that cutoff, something with a higher concentration of poppy seeds still might, like a poppy seed paste. And the SAMHSA cutoff is a recommendation; if you’re a government employee, your tests should follow that guideline. But other drug test administrators are under no obligation to adhere to the SAMHSA regulations. Treatment facilities or doctors’ offices might use lower cutoffs, making their tests more likely to detect the consumption of poppy seeds.

False Positives on Instant Urinalysis Kits

Poppy seeds aren’t the only substance that might trigger an unmerited positive on some drug tests. Immunoassay tests, the kind used in most instant urinalysis kits and as a preliminary screening tool in the lab, are notorious for picking up false positives from common medications. These include antihistamines, antidepressants, antipsychotics, antibiotics, analgesics, and other over-the-counter medicines. Specifically included on the list are ibuprofen, dextromethorphan (an ingredient commonly found in cold medicine that has its own intoxicating properties), diphenhydramine, pseudoephedrine, and ranitidine (an antacid/antihistamine). These drugs can cause positives for different substances, including THC, opioids, or benzodiazepines, but the most common false results are amphetamines.

Positives that result from poppy seeds are tricky because they are, in a sense, genuine positives. Your body has, in fact, metabolized an opioid alkaloid; it’s just that it didn’t come from an illicit source and it wasn’t in quantities that could produce an intoxicating or euphoric effect. But when a positive for methamphetamine is triggered because you took some cold medicine, that’s a false positive—and that can be determined conclusively by further lab testing.

Marino says that many of these substances are structurally very similar, “so it makes sense that antibody tests can’t tell the difference... but if you send it out [to a lab] for gas chromatography-mass spectrometry or liquid chromatography-mass spectrometry testing, that would be able to pick up most of these compounds.”

The only issue here is whether whoever is testing you is willing to send the sample for another test. If you’re being tested on-site for a job, it’s entirely possible that your employer does not have a system in place for sending your sample to be examined in a lab. So you should definitely tell your employer in advance of the test if you have taken any medications. Hopefully, if it’s one that could trigger a false positive, your employer will give you the benefit of the doubt.

What About CBD?

Another substance that trips people up is cannabidiol (CBD). CBD is the non-intoxicating chemical compound found in the cannabis plant, which is generally credited for many of the plant’s medicinal properties. CBD was recently approved by the FDA to treat seizures and is marketed as a medicine called Epidiolex.

But you don’t have to be prescribed Epidiolex to get your hands on CBD. It’s sold in a variety of stores and can often be found in smoke shops, vape stores, and recreational marijuana shops. People often wonder, however, if CBD can trigger a marijuana positive on a drug test. The simple answer is no: Drug tests look for THC, the intoxicating ingredient in marijuana. They don’t test for CBD, so CBD won’t make you pop positive for THC.

The reality is a little more complicated. Because CBD is derived from the same plant species as THC, trace amounts of THC can end up in your CBD product. In order for CBD to be (mostly) legal, it has to come from a hemp plant (and there’s some weird politics around even that). That means the plant can’t contain more than a trace amount of THC. So if your CBD is coming from a hemp source—and if you’re buying it from a non-medicinal source in a state that has not legalized recreational marijuana, it probably is—then it's unlikely to contain more than a trace amount of THC. And that should not show up on a drug test.

But you do need to be careful to check your sources, especially if you’re buying from a rec store. Some companies intentionally add small amounts of THC because they believe it potentiates the therapeutic effects of the CBD. Those small amounts can range from 1 percent to 15 percent—and that amount can be detected in a urine test. It’s not a false positive, either. Even if you didn’t “feel” the THC, you still consumed it. So you won’t have much ground for disputing those results. Basically, if you’re going to use CBD, check your sources and make sure the THC levels fall below 0.3 percent, which is the legal limit for a hemp product.

Drug testing is a politically complicated practice. Many people find it degrading, or feel that it adds an unnecessary element of surveillance into their lives. Nonetheless, if you find yourself in a position in which you have to take a drug test, it’s important to understand how and why a positive could show up even when you haven’t consumed illegal drugs. Bottom line: If you know you’re going to be tested, skip the poppy seed muffin.


Have you ever gotten a false positive? Give us the details in the comments.

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Elizabeth Brico is a writer from the Pacific Northwest. She got her MFA in Writing & Poetics from Naropa University, where she justified spending more time shooting dope than doing homework because William Burroughs once taught there. Now, she writes about trauma, addiction, and recovery on her blog Betty's Battleground. She's also a regular contributor to the PTSD blog on HealthyPlace, and freelances as much as she can for The Fix and Tonic/VICE. Her work has also appeared on VoxStatOzyTalk PovertyRacked, and The Establishment, among others. In her free time she can usually be found reading, writing, or watching speculative fiction. Find her portfolio and ramblings about writing on eb-writes.com, or stalk her on Twitter: @elizabethbrico (if you're interesting, she might even stalk you back).