Medically Reviewed by
Jason Crawford

Article Last Updated on January 3, 2023

Concerns about second-hand smoke are common among people, especially for those who are exposed to occasional unannounced drug tests at their workplace. People who live with a marijuana user who takes cannabis either recreationally or as medical marijuana may often wonder whether their marijuana use can affect them somehow or cause them to test positive on a drug test.

So can you fail a drug test from secondhand smoke? There have been many questions like this (as well as misconceptions about weed smoke exposure) so to find out how all this actually works and if second-hand weed smoke can get you in trouble, keep reading.

What Are the Health Effects of Second-Hand Weed Smoke?

We all know how harmful getting second-hand tobacco smoke is – it’s been talked about for decades. With the combustion of tobacco and the other substances present in cigarettes, many harmful chemicals are released. These compounds irritate the throat and the lungs, making it worse for non-smokers than smokers. But what about second-hand cannabis smoke?

Generally speaking, smoke is not very friendly to your lungs. However, the spectrum of how bad it is also depends on each person’s individual tolerance. For some people, it’s not an irritant at all, and others, who have health issues, could feel aggravated right away.

There’s another tricky aspect here, though. While pure tobacco is generally smoked in the form of paper rolls or in the form of cigarettes that contain other substances, weed offers more options to cannabis users than just a joint. You can use a pipe or a bong, which gives you a smoother and cleaner smoke, as well as vaporizers that don’t combust the plant material.

One More Thing

The effects of second-hand marijuana smoke have not been examined in detail in circumstances that are true to life. One animal-based study, though, has examined the effects of THC-free smoke on mice and found out that second-hand pot smoke shrinks blood vessels in a similar way to tobacco smoke. However, there have been no studies with humans and scientists are still looking for definitive answers.

Therefore, it could be said that it’s a grey area as to whether second-hand exposure to marijuana smoke is as irritating as tobacco. For what it’s worth, in order for it to have some really harmful effects, you’d have to be surrounded by smoke for prolonged periods of time. If you hang out with your friends for a smoke session once in a while, it’s unlikely that it’s dangerous.

Can Second-Hand Marijuana Smoke Show Up in a Urine Drug Test?

Standard drug tests look for THC metabolites in the urine, not CBD (cannabidiol), because THC is the psychoactive compound that causes the high. CBD cannot cause a false positive test as it’s not the compound that drug tests screen for.

Drug test results will be significantly different for marijuana smokers and people who have inhaled second-hand smoke or experienced a “contact high” after exposure. Therefore, whether second-hand weed smoke can be detected in your urine during drug testing or not depends on several factors.

What Clinical Trials Have Shown

For starters, it depends on the environment you’re in. A 2015 clinical trial by Johns Hopkins University School of Medicine concluded that being in an unventilated room (a hotbox) for an hour with someone who smokes marijuana can affect you. The study involved 12 participants who sat together in a chamber with minimal ventilation for an hour. During that time, six of them were smoking marijuana cigarettes, while the other six weren’t.

At the end of the session, the participants who weren’t smoking had detectable levels of cannabinoid THC (tetrahydrocannabinol) in their urine samples, meaning it was above the cutoff levels. They also had minimally increased heart rate and reported feeling mildly to moderately woozy. Additionally, they demonstrated impaired performance on a logic performance test that was taken before and after the smoke session.

On the other hand, the same clinical trial had repeated the smoke session, but the second time they did the test in a ventilated room. The urine tests turned out to be negative, meaning the levels of THC were undetectable. The participants had no elevated heart rate and there were no impairments on their performance tests.

Another study shows similar results. A 2015 clinical trial attempted to capture a more true-to-life scenario where the participants spent time together in a well-ventilated place. The smokers and non-smokers hung out for 3 hours after which the non-smokers’ urine was tested. The researchers found only trace amounts of THC, which is not enough to result in a positive drug test. 

What Does This Mean?

The National Institute on Drug Abuse seems to have the same debate. Therefore, there isn’t a straightforward answer to this question, but it seems like ventilation has a lot to do with the detection of THC in urine.

Technically, THC can be detected in urine after passive smoking, however, the tests would have to be really sensitive and be able to detect the smallest concentrations. However, the threshold of the standard drug tests is 50 nanograms per milliliter (ng/mL), meaning that the amount of THC in the urine will have to be above this number.

That said, detecting a certain amount of second-hand weed smoke in urine testing is not impossible, but it’s not enough to fail a drug test. And since ventilation can influence whether you get a contact high or not, it’s best to crack a window and avoid confined spaces (like a car for example) when you’re with someone who smokes weed.

However, if you want to be safe just in case and avoid having a positive urine test, there are ways to eliminate second-hand weed smoke from your system.

How to Eliminate Second-Hand Weed Smoke from Your System

  1. Drink more water. This one may sound like a no-brainer, but it’s really the most basic thing you can do to flush the THC out of your system sooner. To do it effectively, make sure you drink enough water for 24-48 hours to make you pee more often than usual. The goal is to dilute the urine as much as possible, so even if the drug test is more sensitive, it won’t be able to detect any residual THC. However, be careful, and don’t overdo it. If you drink too much water, your urine will be too diluted, and it’s generally a bad idea to stress the body with that many fluids.
  2. Take zinc. Zinc is known as a urinary adulterant, meaning that it can weaken the presence of the THC metabolites, rendering them harder to detect for 12 to 18 hours. You can easily find zinc supplements in any pharmacy or health food store, but if you’re on any other medication, make sure to consult with your doctor first before taking zinc.
  3. Take activated charcoal. It binds to different substances in the body and it helps to eliminate them. Charcoal molecules have a negative electrical charge which makes them able to easily bind with positively charged molecules, like the ones found in weed, and trap them. If you take activated charcoal, make sure to wait for two hours before you take any other medication as the charcoal will hinder absorption.

Bottom Line

There are valid concerns about second-hand weed smoke showing up in urine screening as there have been many misconceptions and myths about it, but no firm conclusions (and scientists are still looking into it).

From what we have so far, it’s clear that being in a well-ventilated place seems to make the most difference, and even if there are trace amounts of THC metabolites, they are unlikely to be detected by standard drug tests.

A passionate advocate for the benefits of cannabis. Fraser Horton, who has a background in botany and a strong love of nature, has spent years researching how cannabis affects the body and mind. He established Leaf Nation in 2020, where he has devoted himself to educating people about the legalisation of marijuana and its safe and responsible use. Fraser is committed to highlighting cannabis’ potential for improving wellness and working to dispel the stigma associated with its use.


The information presented on this page is provided as a public service to aid in education and is derived from sources believed to be reliable. Readers are responsible for making their own assessment of the topics discussed here. In no event shall Leaf Nation be held reliable for any injury, loss or damage that could happen if using or abusing drugs.