What Are the Risks of Secondhand Marijuana Smoke? Drug Testing, Contact Effects, and More

Even if you don’t personally smoke cannabis (marijuana), you might still inhale secondhand smoke from time to time. This is the smoke that comes from burning cannabis, which you can inhale when you’re near someone who’s smoking. 

Experts are still learning more about secondhand cannabis smoke. But they know it contains many of the same toxic chemicals as cigarette smoke. And beyond the health risks, secondhand smoke from cannabis or vape pens can also make you high or fail a drug test. 

So, let’s take a closer look at exactly how your exposure to other people’s cannabis use can affect you.

Can you get a secondhand (contact) high from marijuana smoke?

Yes, it’s possible to get high from secondhand cannabis smoke. But it depends on the amount of your exposure to THC, or tetrahydrocannabinol, the active part of cannabis that causes the high. 

A secondhand high also depends on the amount of ventilation in the room. For example, spending an hour in an unventilated room with cannabis smoke could be enough to get a contact high. And it could also affect a drug test.

Will secondhand marijuana smoke make me fail a drug test?

Possibly. Similar to a contact high, it’s possible for secondhand cannabis smoke to cause elevated drug levels that can cause a positive drug test. But formal studies on this are limited.

One study found that half of children exposed to cannabis smoke at home had positive levels of THC in their urine. 

A very small study in adults also found detectable levels of THC in blood and urine. But the researchers point out that this is more likely if you’re:

• Exposed to high amounts of smoke

• In a poorly ventilated room

• Tested shortly after exposure

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• What causes a false positive drug test? Learn about which medications can cause a false positive result on standard drug tests.

So, while it’s possible for you to have a positive drug test from secondhand smoke, it’s still unlikely.

How long does secondhand smoke stay in your system? 

It’s hard to predict how long secondhand smoke will stay in your system after exposure. This will mainly depend on your level of exposure: how much smoke you were exposed to and the ventilation of the room. 

In the study mentioned above, the one with adults, some of the study participants who were exposed to secondhand smoke still had detectable levels of THC in their urine 32 hours after exposure. But it’s important to note that the researchers used lab tests that were more sensitive than standard drug tests. It’d be unlikely that a normal drug test would be positive beyond 12 hours after exposure.

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What are the adverse effects of secondhand marijuana smoke?

Research is limited on this. But experts warn that secondhand cannabis smoke still contains many of the same toxic and cancer-causing chemicals as tobacco smoke. And we know that secondhand smoke increases the risk of:

• Lung damage

• Heart disease

• Stroke

• Lung and other types of cancer

• Pregnancy complications

This is especially harmful for at-risk populations such as children or those with other chronic medical conditions. 

Still, some new research is looking just at the negative effects of secondhand cannabis smoke.

Lung damage

Any kind of smoke can damage your lungs, including cannabis smoke. The toxic chemicals in cannabis smoke can increase your risk of long-term lung conditions, such as:

• Mucous production and cough

• Breathing problems like wheezing

• Bronchitis

• Pneumonia

• COPD (chronic obstructive pulmonary disease)

Cancer

Heavy cannabis use through smoking can increase your risk of cancer. Smoked cannabis, just like smoked tobacco, contains cancer-causing chemicals. And your risk will depend on how much and how often you smoke. 

A study looking at risk over 40 years showed that heavy cannabis use was associated with more than twice the risk of developing lung cancer. The long-term effects of light or occasional cannabis use are still unclear.

Heart disease

There’s limited research on the cardiovascular risks of cannabis. But some early data shows that exposure to cannabis smoke may put you at greater risk of:

• A higher heart rate

• Elevated blood pressure

• Arrhythmia (irregular heart rhythm or extra heart beats)

• Increased risk of blood clot, heart attack, or stroke

And research suggests that any type of cannabis use can increase your risk of heart attack and stroke. But it’s still unclear how this applies to people exposed to only secondhand cannabis smoke.

Pregnancy risks

Cannabis smoke can also affect a growing baby in pregnancy. Risks can include:

• Stillbirth

• Preterm birth

• Developmental delays

• Growth problems (low birth weight)

Brain effects

Early research has shown mixed results on the long-term effects of cannabis on the brain. Some studies have linked cannabis use to long-term problems with:

• Memory

• Thinking, learning, and IQ 

• Impulse control 

• Mental health conditions and addiction

And these concerns and risks are especially worrying for teens, who start using cannabis when the brain is still developing. Previous studies have found a link between cannabis use and performance on cognitive and memory tests. 

But newer research suggests that cannabis may not be to blame. Instead, the differences in test scores could be due to underlying factors that contribute to someone using cannabis and having problems with thinking and reasoning. These factors are socioeconomic, such as education level, employment status, and income. 

Cannabis can affect how a teen’s brain functions in the short term, according to the researchers. But cannabis doesn’t seem to impact long-term mental health and cognitive ability as previously thought. 

This is an ongoing topic of research.

Pregnancy risks

Cannabis smoke can also affect a growing baby in pregnancy. Risks can include:

• Stillbirth

• Preterm birth

• Developmental delays

• Growth problems (low birth weight) 

Why trust our experts?

Written by:

Christina Palmer, MD

Christina Palmer, MD, is a board-certified family physician with a special interest in chronic care management, women’s health, mental health, and preventive care. She has over 10 years of experience in primary care research, innovation, and practice.

Edited by:

Katie E. Golden, MD

Katie E. Golden, MD, is a board-certified emergency medicine physician and a medical editor at GoodRx.

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